Good info guys. I've since tried both and both work perfectly fine. I have no idea what the problem was, especially since I'd previously deliberately lubed healthily after I couldn't get any resolution.
It occurred to me - is it possible the vent hole in the die was somehow blocked? I blew it out and cleaned the die with solvent and it was after that that things went to normal. I still can't believe the system is so airtight it would do this, or even if it was, that's enough air pressure to prevent the full sizing. I didn't change anything and it now works so my guess is simple friction, though I've never had anything seize like this.