Sorry! I don't know if I can help. I've opened light fixtures less than a dozen times in my pool career, because they aren't used much on the large commercial pools I specialized on. I never had any particular problem resealing them, though I did always replace the gasket. I can think of a few things to check:

1. Make sure water is not entering via the wire seal at the back of the fixture.
2. Make sure the new gasket is OEM, not aftermarket rubber.
3. Make sure that, when you reseal the gasket, you snug all fasteners lightly, and then evenly cross tighten till down.