Possible over pressure situation swelling the brass. From someone that has spent a lot of time and money trying to come up with a 2 oz HS load that is not overpressure, it is tough to move 2 oz of HS from a 3" shell and stay within limits. We have seen the same issues with Nitro shells in the past. It is also possible that you have a rough or dirty chamber that is causing the hulls to stick. But if you shoot a H13 straight 6 or 7 @ 1090 fps and they do not stick, I think you will have your answer.