If there is a bezel around the lenses try tightening it. Also, if you can loosen the leses, try cleaning out the channel they fit into on the mask. There may be sand or other stuff in there.
Have you contacted the manufacturer? This usually the best place to start on any issue you can't resolve yourself. Your LDS may also have some advice or solutions.
For a DIY solution, if you can't loosen the lenses within the mask, would be to try sealing the edge of the lenses with silicone sealant. You may want to obtain the type of sealant that is certified for aquairiums, as there may be some issues having this stuff close to your eyes. Be sure to mask off the lenses with masking tape for a neat bead and easy clean up. Give the material about a week, or more, to cure before using the mask.
