Sometimes I get that with vld-type bullets that have a long ogive that's a hollow-point with no lead under the jacket.
If you're using those bullets and a standard seater plug, the seater plug can compress the unsupported copper a little differently each time. I solved the problem by using a vld chamfer tool on the seater plug, it gives a longer bearing surface to contact the bullet and less variance.
The other thing that can cause that is too much neck tension (stiff seating) and/or no lube on the inside of the neck. If you can get a smoother seating stroke by either less neck tension (not an option with a standard sizing die, you'd need a bigger neck bushing) or lubing the inside of the neck that helps sometimes too.
