Another reason for it is that extractor is just not very strong, so if there's a lot of carbon build up around the rim face or in the chamber itself this can make matters worse. Mine was doing the same thing and I hosed it down with Kroil let it sit and went work with a tooth brush and bore mop. Finished it off by hosing it down with carb cleaner and a good oiling of what needed oiling and she extracts like a champ.
