Subujee, everyone is correct. The mask needs to form fit the face to keep it in place. In my case they actually did redo it for me after a few days. Can't remember why but the second mask seemed to do the trick. But even then it was pressing down so hard on my - let me be generous to myself and say magnificent instead of merely prominent - nose that the tech cut a circle out of the mesh to allow that glorious ship's prow to cut through the waves of air as it was meant to. Which helped a lot and apparently made no difference to the positioning of my head.

If your father is having a problem in this department you might ask about a snip job! (er, of the mask, not his nose)

D2