I had substantially the same problem when I started, and found that if I broke down the mask clearing into three separate actions I had MUCH better success: 1) Press on the top of the mask with face inclined toward the surface a little, then blow out through the nose, 2) Wait a second and make sure you are set up to breath through your mouth, 3) Breath through your mouth (repeat from 1) as necessary). I now don't even think about this. Incidentally, I was a wind musician for many, many years before I started diving, and so I thought I'd have pretty good control over when I was breathing through my nose and when through my mouth...just goes to show how unnatural mask clearing can be at the beginning. Also, on mask removal and replacement, don't rush the clearing. Take your time when putting the mask back on, and concentrate on just the breathing and maintaining your bouyancy. I didn't have this problem, but my son-in-law did when he first started.