A few things that could be going on here.

What dies are you using?
Are you seating and crimping in the same step?

I notice a noticeable difference while performing plunk test with seating in a separate step. It even allowed me to load longer with the same bullet. Plated bullets did not behave this way. I believe it is the lead getting squeezed/ shaved out a bit while seat/crimp at the same time.