It may be because you installed it too low.
On the inside of the tank where the base meets the sidewall, there is a curved surface. Fittings do not seal very well on curved surfaces. If you install your fitting too low, it will hit this curved surface.
The only fix for this is to take the fitting off, try to grind the curved surface on the inside of the tank to make it as flat as possible and then reinstall the fitting.
Tip: Always recommend installing the bottom fitting up a little bit to make sure it is on the flat surface on the inside of the tank.