You may have the size button adjusted too high in the die, i.e. it's too close to the neck sizing portion of the die. Then when withdrawing the case, you have part of the case neck still in the neck sizing portion of the die while the lower portion of the case neck is being forced onto the size button.
Try adjusting the size button a bit lower in the die, but not so far down it impacts the interior of the case.
Also try sizing a few cases with the decapping rod removed from the die. It won't hurt a thing.
