I have had this issue as well. I found that using an overshot card added enough height to get a proper crimp. If adjusting the final crimp down a bit causes the crimp to, basically, go down into the shell the load is allowing to much space above the it to get a proper crimp. I have also switched to a heavier load (shot wise) and wads that a a bit taller.

Just my experience....

WBLDon