You could have one of a million things going on. Usually, its a technique/tightness issue. When the weight gets heavy, form breaks down. It happens. The difference between getting the weight and not getting it is your ability to stay tight. Work on your upper back strength and focus more on 'pushing yourself away from the bar' rather than just benching the bar back up.
Another common problem is a shoulder capsule imbalance. The thick tendonous sac that connects your upper arm to the res of your body can get thrown out of whack from lots of heavy pressing. Check out www.mobilitywod.com, do a search for 'shoulder' and do every exercise that pops up.
Hope this helps. Solid press by the way.