Are the bulelts loose before you crimp them? If so, its a resizing/bullet problem.

If they are tight before you crimp, you might be crimping them too hard, squeazing the bullet diameter down a bit and the brass springs back, leaving you with a loose fitting bullet.

The crimp you use for a semiauto (maybe revolver round as well, but I don't know as I don't reload for any revolvers) is to remove the belled mouth of the case only, not to hold the bullet in place.