This was written for the Python twenty years ago. The Webley lockwork is very similar:
Part I. How the Colt I & D Frame works:
In the Double Action mode, as the trigger is pulled rearward, the hand is raised. The finger of the rebound lever rests on the pivot pin of the hand. So, as the hand rises and engages the ratchet of the cylinder, it carries the rebound lever upward and causes the tail of the bolt to pivot upwards. Resultingly, the bolt drops, disengaging it from the cylinder and allows the cylinder to be rotated by the hand. Concurrently, the safety lever is raised (by the trigger) causing the safety to slide downwards. The trigger nose now begins to engage the sear, allowing the hammer to start its rearward motion. When the hand rotates the the cylinder 1/2 to 2/3 of the point of indexing the next chamber, the bolt tail falls off the rebound lever shelf. The bolt is then forced upwards by the bolt spring, resulting in the bolt engaging the leading notch of the cylinder. The cylinder is then arrested from further rotation by the bolt which engages the cylinder notch. The trigger nose continues to raise the sear. When this happens, the hammer is forced forward by the pressure acting upon it from the rebound lever. The mainspring supplies the pressure for the rebound lever. The hammer falls on the frame mounted and spring loaded firing pin, which strikes the primer; thereby compressing the anvil in the primer. The crushing of the anvil grinds the primer compound to 300 degrees, thereby causing ignition. The flash travels through the primer hole into the chamber of the case, igniting the propellant. The gases formed by the propellant dislodges the bullet from the case. The bullet then begins its journey which will take it from the cylinder, past the barrel cylinder gap, through the forcing cone, down the barrel and eventually out of the muzzle.
As the trigger is released, the hand begins to lower and disengages itself from the cylinder ratchet. This permits the rebound lever finger to follow the hand downwards. As the rebound lever lowers, the shelf of the rebound lever forces the bolt tail raised. The trigger nose pushes the sear inward (towards the hammer) and an audible click can be hard as the trigger disengages the sear and allows the sear to return to its position of rest. The safety lever is lowered, allowing the safety to rise. The hammer is pulled away from the firing pin by the mainspring. The bolt tail now pops back onto the shelf of the rebound lever and another audible click is heard. The trigger is now returned to its position of rest by pressure from the finger of the rebound lever. The hammer returns to its position of rest by both the mainspring and a camming action from the rebound lever against the seat of the hammer.
A word of caution on working on your own Colt I frame gun. Some guns have parts which performs a single function. On the Colt I or D or E frame guns, a part may perform numerous functions, all of which relates to timing. Changing one part may affect the timing of several other parts. This is especially true of the Rebound Lever. So, whereas one problem may be corrected, two or three others may have been created. Also, consider the cheapest alternative first which allows for restoration to status quo ante. Bending a part as a solution which you find doesn't work may be resolved by bending it back to its original shape. Removing metal may sometimes be remedied by lengthening by peening; the operative word is sometimes. Since Colt uses 4140 steel for its parts (on D, I and E frames), this allows for peening and shaping by filing/stoning to restore function. On a Colt, you should rarely need to replace parts. If a task appears too daunting, send it to a gunsmith.
So, with those words of advisory out of the way, let us begin by defining a common thread: definition. Unlike the cylinder stop of a S&W or Ruger revolver, the bolt on a Colt (D, I and E frame) is rather lengthy. We'll refer to the "cylinder stop" portion of the bolt as the bolt head and the opposite end which rests on the rebound lever shelf as the "tail". The actual tip of the tail will be referred to as the actuator.
With that, let's look at some ways to delay the bolt from popping up too early.
Recall from Part I how the hand raises the rebound lever and that that action causes the bolt to pivot downward and disengage from the cylinder notch. If you increase the time required for the rebound lever to rise, you delay the bolt's pop. This may be achieved by removing metal from the radius of the pivot pin of the hand. We're talking about removing metal from the top where the rebound lever finger rests on that pivot pin. The end result is that the rebound lever sits slightly lower, thereby taking more time to rise. Doing this should not affect the dropping of the bolt.
Another alternative is working on the bolt itself. As you recall, the bolt tail rests on the rebound lever shelf. If it's been bent previously such that it cants away from the rebound lever shelf. From a top view looking from the top of the gun towards the bottom of the grip, if the tail appears bent towards the solid side of the frame as opposed to the sideplate side, then the bolt tail and actuator sits more precariously on the rebound lever shelf, shortening the time it will rest on that shelf before dropping off (and allowing the bolt to pop up). The bolt may be placed in the vise and with only the tail exposed, grasp with a plier and tweak it (top view again) slightly to the left. This will increase the actuator's contact with the rebound lever shelf, thereby giving you a little more time.
Another thing we were taught was that the main spring could have been weakened, and that increasing spring tension could affect timing of the bolt.
I've also worked on a Colt Official Police. It has the same lockwork as the Python. It needed a new bolt and after it was fitted, the gun had more issues. The trigger was not returning because the bolt wasn't climbing back on the rebound lever shelf. I bent the rebound lever and then polished part of the shelf. I also polished to bolt tail and that only helped a little. The hand was polished to remove any scratches or burrs. My teacher diagnosed the problem back to the rebound lever. The rebound lever also pushes the hammer back, causing the firing pin to retract from the primer. It does it two ways. First is the mainspring which helps pull the hammer back. The second (and why it hung up) was that it pushes at the back bearing surface of the hammer, causing it to rotate back to its position of rest. The rebound lever had been worked over, removing too much metal from it thus not having the surface area to push on the hammer. The cheap quick fix (because no new rebound levers are to be had) was to file down the hammer block at 45 degrees, leaving only 1/2 of its contact surface to block the hammer. Now on Monday I have to lengthen the hand so it will rotate the cylinder earlier.
Finally, if these measures don't work, then fitting a new bolt may be in order. Alternatively, the rebound lever may have to be replaced (and may have been the culprit all the time). If it comes to either two, I'd recommend sending it to a gunsmith or back to Colt.
Notes on the hand: Like the Colt SAA, there are two steps in the hand. The top step initiates the rotation of the cylinder and the lower or second step completes the rotation. The top step should be inclined towards the center pin. If it doesn't, it can cause drag when the second step takes over the rotation. Also, the lower/second step should also be slightly inclined towards the center pin. The belly of the hand should not be so fat so as to push the first step away from the ratchet. With the side plate off, you can watch the hand move as it enters the hand window. It should begin from a retracted position and as it rises, then engage the ratchet. Near the hand's pivot pin you will notice that there is bevel for the finger of the rebound lever. If the two mate perfectly or close to perfect, then the finger will drag on the hand and could possibly prevent the trigger returning to position of rest when the trigger is release (sticky trigger). Polish the finger (not file) and you can gently file to alter the angle of that bevel on the hand.
Cylinder Disassembly: You need a special tool to grab onto the rachet. Brownell's sells it. Push the extractor to give full exposure to the ratchet. Then rotate it so you can unscrew the ratchet. Remove the ratchet. You now need another special tool to disassemble the cylinder screw.