Any kind of sealant or lubricant on the rubber gasket tends to make to creep or squish out of place.
Always install bulkheads DRY, put the rubber gasket on the FLANGE side regardless if it is on the inside or the outside. Never hang any plumbing off the bulkhead unless it is supported in someway. Plumbing can cause unequal stress which leads to leaks. Hand tighten and maybe 1/4 turn past that but no more, overtightening is probably the number one source of leaks. If using threaded bulkheads always clean the flashing or excess plastic out of the threads on the fittings, this tears or smears the teflon away when tightening if not cleaned up. I prefer the putty sticks from LaCo or Ace Hardware over teflon tape. It was sugested to me years ago by my swimming pool plumbing contractor as its the only thing they ever use and my pool has never had a plumbing leak since 1983 and the fittings still have pliable putty on them every time I have replaced/repaired a pump.
Make sure you have the correct size bulkhead for the hole, little to no slop is best as that gives more sealing surface for the gasket. Bulkheads are not all the same size, it varies by manufacturer.