I'm using a MEC 9000 progressive machine. Using AA hulls. About 1 in 50 shells ends up with a crimp that won't stay closed. It isn't apparent immediately, but happens over a couple of days. I don't see it when I'm putting the shells in the box, but a couple of days later I notice a "bulgy" spot in the box and find a shell that has a open, or partially open crimp. I hate to make any adjustments on the machine since it has been behaving particularly well for a few months. Any ideas?