I'm reloading 9mm Luger. I resized the brass and primed them as well. I've got the expander rod in the press and I'm opening up the mouth slightly so it will accept the bullet. When I expand the mouth opening the side of the piece of brass ends up with a slight crease on the wall of the brass. I'd describe it as a slight bulge on the sidewall of the brass. It's happening to two out of three pieces of brass. I have no idea as to why the side wall of the brass is getting creased. Any help appreciated.