When Paul asked if your anchor point was consistent, you should be aware that a change in your anchor point of as little as 1/8" can move the point of impact by 6" at 20 yards.
These are other things that affect up/down point of impact:
If your string elbow moves up, the impact point will move down, and vice versa.
Changing the relative pressure of your fingers on the string will move the impact point up or down.
There is a tendency, which may be exaggerated on low draw weight bows, to relax the drawing muscles, whether arm, shoulder or back, at the moment the arrow is released. When these muscles are relaxed a split second before the arrow is released, the arrow will generally fly high. More than a split second and the shot collapses and the arrow is generally plucked, resulting in a miss to the right. The solution is to keep pulling through the shot, so that these muscles are not relaxed until AFTER the arrow is released.
A change in draw length from shot to shot will cause high/low misses. This can be caused by extending the bow arm out more or less from shot to shot, changing the alignment of your shoulders from shot to shot, drooping or not drooping your head from shot to shot, changing the angle that your head is rotated toward the target from shot to shot, or changing your posture from shot to shot.
Arrows of different weight will impact high or low.