A enlarged flash HOLE and carbon build up in the flash CHANNEL can cause excess pressure on the primer. If its an original Lehigh bare primer conversion, the vent liner can be replaced if needed.

I would also try a Win209 primer. Its all i use in my Lehigh NFPJ conversions. You can also dry fire the primer. That will push the primer anvil back into the cup a little and then the primer should fall out of the NFPJ adapter.