I use teflon tape on everything but gas pipes, and I've never had a
leak. Are you putting on enough layers of teflon tape on the threads?

For gas lines, I use conventional pipe dope, because the contained in
the distributed natural gas seems to bond better with black iron gas
pipe and pipe dope than with teflon tape. Also, this was the advice
that I received from a friend who works for the local gas company.

Harry C.