[Omathisen]

Left my old Compomotive wheels at a garage this morning to have new tires put on. The mechanic calls me and tells me the center piece is "frozen" and they cannot get the wheels on the machine to balance them.
I am not a wheel or tire expert, but is there a different way of balancing them? And/or a trick to prevent the centers from "freezing."
They are my old 16 inch rims, and I am waiting for new 17 inch rims from Vintagewheels, but had hoped to keep my old ones for spares.
Anyone with similar experience or solutions?
Thanks.