how loud is your bow? how close are the deer when you are shooting at them?
there are 2 things that come to mind from what you are describing: the deer is jumping the string or is so close that your 20 yd pin shoots too high at 10 yards.
also, a 20 yard shot from a 18 foot high tree stand looks like more of a thirty yard shot because you are so far up in the air...but you have to measure the distance from the base of the tree to where the deer is to get the effective yardage. so if you over estimate the distance you will shoot over the deer.
estimating distance is the most underrated skill involved in deer hunting and is something that most ppl dont practice. most ppl shoot at premeasured distances that their pins are set at and then go out in the woods and cannot hit anything cuz they dont know how far their target is...