Rodd,
Kind of guessing here so take it for what it is worth. I wonder if as you practice from the chair, if you aren't letting your torso relax too much and that in turn is not allowing your shoulders to align with the shot? If you are used to a standing shot, you automatically have more core tension with which to turn your shoulders. Now when you sit, you don't (or might not) have that and IF your shoulders are open to the target, you kind of have a built in collapse.
This is mostly a guess -- not actually seeing what you might be doing.
Arne