McLaws and Walker hit Sedgewick's isolated II Corps Division ( accompanied by II Corps commander E. V. Sumner ) as it entered the West Woods. The cornfield position had already largely been given up when G.S. Greene's XII Corps division successfully held the Dunker Church until after Sedgewick was in turn broken anbd driven back from West Woods, exposing Greene's right and compelling him also to withdraw back to ( but not through ) the Cornfield, which remained in Union hands.
I think from reading your question, you have a tendency ( natural ) to think of these units in terms of solid wholes, like I Corps, McLaws' Division, etc., when in fact they were more often fragmented and entered the battle piecemeal. When Sumner accompanied Sedgewick into the West Woods he effectively gave up what is called command control, having no effect on the other 2/3 of his corps; they entered the fight separately and much farther south of Sedgewick so were no longer in supporting distance. Under Richardson and French they made separate and uncoordinated attacks on the Sunken Road. The death of untried XII Corps commander J. K. Mansfield early in the battle also robbed that corps of command control.
It's also well to remember the unequal sizes of these elements: The Union IX Corps had 4 divisions to deploy; the XII Corps only 2, though 3 was the norm. Likewise, Confederate divisons and brigade sizes varied widely: A. P. Hill's so-called Light Division was twice as big as many of the others, notably undersized ones like Walker's and Hood's.