Are you bending at the waist in order to keep your anchor points the same as standing flat on the ground? This seems to be the most commen reply to the treestand question.

The best thing to do would be to practice out of your stand at tree stand hunting height and shoot at distances varying from 5 yards out to what ever your max range is.

I do this every year before season in order to boost my confidence. My pendulum performed well this year out to 25 yards, it then dropped to shooting ~4" low at 30.