Normally, seating gas checks on cast slugs is pretty straightforward.
You install the gas check, give the bullet a couple sharp "raps" on a hard table surface, and run it through the sizer-lubricator.
If you are tapping the bullet NOSE first into the Lee sizing die, that might be the problem. The tapping tends to jar the gas check loose before it is crimped onto the base. Are you pan lubing and using a hand sizer die?
Most lubricator-sizers push the gas checked bullet BASE first into the die, and that tends to crimp the gc on. No tapping to jar it loose either.