Because the brass is considerable harder than the Cooper jacket it will shave the bullet in some instances. This is not a concern the allowable can't in the stem to bullet relationship would be by guess to what's causing your problem. You may be able to set the seating die all the way down and the stem as high in the body as possible to your desired length to mitigate as much can't as possible. In my ecotone if that doesn't work try it the reverse. Die high stem low so you can guide the bullet as long as possible.