I've got an old Stevens 20 gauge break open that is having extracting issues. It readily ejects an unfired short base round out crisply to 2 feet or so, however once fired I have to use a rod to bump the hull out as the extractor stays in place. I've done the same thing with high base and it will actually eject the hull but not as well as an unfired round. I've cleaned the chamber and even buffed it a little with liteweight emery paper. On inspection I see no issues in the chamber as the unfired shell is free moving and the extractor spring is strong. Any ideas on what the problem is and how to cure its ills?