I have recently become more aware of the importance of having the right "fit" of your personal mask to your facial structure. The mask may be sealed, but if it is the wrong size for your face, then it will leak as you move around, moving your head to different angles, as you work.
I have also read that you can't effectively use a gas mask if you have a beard; you will not be able to get a good seal where the facial hair meets the mask.