Are you a handloader? If so, you might have an issue with the brass getting stuck in the chamber, possibly needing a full-length resize on the brass. I once had some 7.62x39 brass that kept sticking. Come to find out, I had reloaded it so many times that it wouldn't take a good resize, thus causing to to stick in the chamber almost every time.
You might look to make sure that the chamber isn't dirty, this will cause them to stick occaisionally. Give it a good cleaning and see if that helps.