I have not had that problem, but I am only casting for pistol bullets right now.

I've only got three or so Lee gc moulds but all mine fill out fine for the gas check. I'm not sure they are quite as good as some of my Lyman moulds but that differs from mould to mould. I'll bet Lee would send you some more if you sent they back. I've not read of the gas check not clamping on with Lee moulds unless your mould is not filling out correctly. Sounds like you need to add some tin to your melt. BM

Bill