Your form could be the culprit. What jumped in to my head was parallax. Are you adjusting your parallax when moving from 100 to 300 yards?
Also, some loads will just not perform at longer ranges. Some will and some wont.
Have you tried different loads or just one?
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.