Anybody have an idea why my H&R 12 firing pin will dent the base of the cartridge but won't set it off. This happens off and on. Some cartridges I have hit 3 or 4 times before it will fire. This happens with all 8 different ammos I have tried. Just bought this gun a couple of weeks ago so I do not know much about it. Any help will greatly be appreciated. Thanks, BILL