Sounds like the expander is too large. When you push the brass upward into the die, the neck is squeezed down, On the reverse - down stroke the expander bead pulls up through the neck, sizing it back to what should be a snug fit on the bullet. If that is too large, say .311 instead of .308, bullet will fit loosely. May be manufacturer out the wrong one in.
I like to use Lee neck sizing die, which uses a collet to squeeze the neck down against the center - anvil.
You can get some real expert answers on reloadbench.com - bulletin board.