John Schulz tells the story, either in his video or book - Straight Shooting of a friend who shot over a bear and didn't know why. John's suggestion was that he cant the bow a lot more and this would eliminate that tendency. Long story short - the guy did and it worked. I find that also. When shooting sitting - butt on the ground, or sitting on my 3 legged stool, I shoot much better with the cant almost horizontal.
Try that and see if it helps. I think the reason, at least for me is I get a better visual perspective of my arrow - as I shoot Hill - split vision. Also, make sure your focus is on the target-spot and not on your arrow or back and forth.
Other than that keep your same form and practice from these various positions a lot.