Hopefully StrangerDanger or one of the other experts on here will reply to you. I'd make sure your shells aren't getting hung up anywhere, polish the tube inside with steel wool but don't lube it. There is a thread on this forum on how to do that. Get some good, heavy dummy rounds to cycle through and see if you can figure out the problem. My GUESS is that the 7 round tube isn't quite flush and the shells are getting hung up there or your spring isn't pushing them out with enough force or both. Your problem isn't normal and you can fix it, you'll learn a lot doing it as well.