If your bow is shooting at a reasonable speed, there shoud be very little difference in the impact between 15 and 20 yards anyway. That is about the difference between shooting a 20yd. level shot, and one that is downhill, but looks like the same distance. Our club has a 15 ft. treestand for practice, and there really is not much difference in the impact between a level 20 yard shot, and a 15 yard treestand shot. The main problems are like Arthur and others stated, maintaining your form when shooting downward. The other thing that may be happening is that you are not following through your shot the same way that you do from a level shooting position. Most people, when leaning over to shoot down, are conscious of bending, and may be in a hurry to "straighten up." If this starts to happen before you have followed through properly, you can also shoot high.