Lloyd Smale,
I had similar issue with LYMAN 41 caliber gas checks, they seemed to be belled excessively and oversized....and that was being used on a lyman 410610 mould. I found two things that helped, first was to size them nose first in my Star sizer, that helped and the second thing that helped out even more was annealing the gaschecks before using them. Those two thing made them stick better to the bullets base.
Frankly, I use Lyman gas checks in 30, 35 and 44 caliber on all sorts of bullets, Lyman, RCBS, LBT, NEI, Mountain Molds and a Hoch. I have no problems with them in those calibers, but in 41, for some reason, they just don't seem work well out of the box, even on Lyman moulds. Fortunately, my 41 itch got scratched and hasn't recurred since I got my 357 Max.
regards,
Graycg