My two cents.
How far are you up? 14, 15, 20, 25 ft. ?
The higher up you go the steeper you angle. The vital section on the animal gets smaller as well. If you are overcompensating by aiming low it will be even more so at higher stand heights cause the arrow will have less "arc" than from a lower elevation. This also will become more acute the closer the deer is to you.
Do you have a good idea of the ranges you are shooting? 10, 20 , 30 yards? Realistically and depending on you comfortable shooting diastance 20-30 yrds is the max.
Do you step off trees or other objects around your stand so you have a pretty good idea of how far you are actually shooting. Maybe tie off different colored ribbons on different trees at different yardages to help you remember how far the deer really is when it does come walking through.
Things look different from treestands than on the ground. To me they always look farther than they really are. I have had problems of overshooting in the past which is opposite your problem.
Someone mentioned getting a range finder and I highly recommend that. I got one this year and when I have remembered to use it I have been dead on.
I've missed more than I have connected so hang in there. Don't get discouraged and learn from your mistakes.
Veni, Vidi, Sagittam Mittere, now I'm ready for my nap