It could be that the heavy arrow flight at the longer shot is causing your brain to want a a faster and flatter arrow flight. If I am having trouble getting a full draw, one cure for me is to take out my Alaskans with the heavy heads and take a session at forty yards. Something about seeing those nose heavy lead balloons come falling out of the air stimulates my brain into going for all the power I can. For over stretching and under drawing I also practice drawing and anchoring without shooting with an arrow that is either net length or with a broadhead that I pull to my finger. For longer arrows I used to wrap some tape around the arrow as a draw check on at least one of the arrows in the quiver.
Mathematically and mechanically the trajectory of a heavy arrow reaches a point where the ratio of the parabolic curve in the arrow flight becomes more obvious, it is the point where range estimation gets very finite. Since, penetration has never been a problem for me and I am a still hunter or ground sitter, I have gone to less gpp with less weight up front, tapered cedars, and enjoy less of a noticeable change on those thirty yards plus shots. I still prefer to shoot deer at 15 feet however.