[Unclenick]

He said it was new brass. Probably the main cause is new brass (and ss pin wet-tumbled brass) have no carbon inside the necks to help provide slip lubrication, so the brass rubs off onto the mandrel and the broken neck is sticking to that build-up. Just dip the necks in some Imperial Neck Lube or some motor mica same as when resizing cases conventionally. If that doesn't stop it, you can polish the mandrel and clean it and burnish graphite into it with a cloth and some alcohol.

A completely different possibility is the collet fingers are locking closed on the neck. If the necks are on the thin side, this is more likely to occur. Take the cap off the die and drop the collet closing female taper into your hand and apply a very thin layer of grease to the inside of it. Teflon grease is good for this. Put it back together and try again.