Sometimes NPT fittings are too long and you can't get enough turns on them for a tight enough fit. They bottom out first. How snug is it when you are tightening it down, does it feel like it's bottoming out? How many threads are still showing once tight? If too many carefully cut two or three threads off the end and file it down nice. This will allow it to thread in two or three threads more where it's wider for a tighter fit.
