See if it will close with a shell in only one or the other chamber. Maybe the problem is at least just on one side. I guess you are sure you're using the right length shell? If the chamber is 2 1/2" and you're trying to load 2 3/4" the shell may seem to fit, but may be getting stuck in the forcing cone. I'd want to be very sure what the problem was before you start altering the chambers...Geo