Try rotating a spent case and see if there's one spot where it will go all the way into the chamber, that would indicate one of the conditions Coyotejoe mentioned, either an egg shaped chamber or the barrel to breech isn't square. In either case, FL or partial FL sizing should solve the problem. A chamber cast would tell you more, your smith should be able to do that although you could too, it's easy with Cerrosafe, see the FAQs for info. You can check the barrel to chamber fit with feeler guages, check on both sides to see if there's more of a gap on one side than the other with the barrel locked up on an empty chamber. Just place the blade tip flat against the standing breech at the right angle to the side of the frame and close the action on it, start thin and work up until you feel resistance when you pull it out. It may lock up tight on .002", and be slightly more loose on one side than the other on a .0015" blade for example.
Tim