You're not the first one to have this problem. Some .45 ACP brass is noticeably thinner than others, and can cause this problem with some bullets. Remington brass comes readily to mind. Additional crimp is NOT the answer; you need to increase the neck tension. If all of your brass suffers from this problem, I would suggest either turning the expander plug down .001-.002", or perhaps trying a different set of dies. If it's just one brand of brass, then you can either avoid that brand or try what I mentioned earlier.
Many, many reloaders seat bullets in one step and then crimp in another step. When you think it through, this process really makes more sense than trying to seat and crimp at the same time.
Don't give up! Try changing one thing at a time until you solve your problem.