Double up the gasket use a thick gasket or use sealant on both sides of the gasket.
This happens all the time . For some reason the mating surfaces arn't prefect.
Many times after a few months it will stop leaking its self.
When I tune a clutch and take the cover off, put a new gasket on 50% of the time it will leak 50% it won't so when it leaks I remove all the oil from the gasket and surfaces with solvent or get a new gasket and then thin coat first side with RTV sealant and stick gasket on perfect and let it dry, Then when installing coat the other side . Done