There are many reasons why a mask might leak. Even the best scuba diving masks can leak! One of the most common ones is that the straps on your dive mask are tightened too much. It might sound strange that you can prevent your dive mask from leaking by loosening the mask strap but when you think about it, it does make sense. If your straps are too tight, it pulls the dive mask too hard against your face. This squeezes the skirt and prevents a tight seal. The straps should only keep the dive mask from falling off your face. They are not there to make a seal. The seal is produced through the feathered edge of the skirt adhering to your skin. If your dive mask leaks because the strap is too tight, then tightening it even more will result in even more leakage. When you end your dive and your mask and skirt leaves an indentation around your face, then you most likely had the strap too tight! Many divers end up tightening the straps too much as a somewhat loose strap can move around when you enter the water. If that's the case, then hold the strap on the back of your head when you go into the water to ensure it stays in place. If your leakage is severe and not related to straps being too tight, then it might just not be the right fitting mask for you. In this case, you might have to invest in a new one that fits your face better.