How to get thermostatic mixing valve to seal

[PythonCrazy]

I fitted a thermostatic mixing valve under a bath (first time outside of training) but can’t seem to get the larger inner connector to seal. It’s got a rubber gasket that I’ve checked is lined up but am still getting a tiny seep of water when fully tightened. Is it ok to add Jet blue or LSX to the thread to get it to seal? I’ve previously seen it advised against in joints with a rubber washer but am now out of ideas.