The proper crimp is all about the proper stack up inside the case. Powder plus the wad leaves so much space for the shot. If when you add the shot, if it does not come up high enough, the crimp will fall in. If the shot comes up too high, you will not get a good fold.
There is a better wad for the job. The trick is coming up with the best one. Usually the wad supplier can guide you to the best one.
To limit your problems, just use one type hull. Each hull type will require a different wad.
