First, did you thoroughly clean out all gas from the threaded hole in the flange and clean the threads with carb cleaner or the like? This is important. Then you need to coat the threads with the sealant and allow it to become tacky before threading the fitting into the flange. After that let it set up for 24 hours before exposing it to gas or running the bike.
The other thing to check is if you actually have cracks running down into the threads on the flange. If so you may need to use a heavier duty sealant. Either Permatex 56521 or even JB weld. If you run out of the sealnt we provided the Permatex is a good alternative. Hope this helps,