@LadyShadow, this makes me think you have a full face mask: "...my mouth is hanging open and the masks slips off above my chin"

Would you be able to use a chin strap with your mask? It should keep your mouth closed.

I use a nasal pillow mask; before that I used a nasal mask. So, no, I've never had the this problem.

But I do seem to have a problem with my mouth open, and the positive pressure escapes out my mouth, which makes the therapy less effective.

The varieties of masks are described here:
https://www.mayoclinic.org/diseases-...s/art-20546828