Seal fail, dry mouth, random mask removal at night

I've finally gotten back to sleeping with my bi-pap. I use the Resmed Liberty Mirage full/hybrid face mask, size small. I am a stomach sleeper and never have a problem keeping a good seal with my face smashed into the pillow or mattress.

In the last few days I have found myself rolling on my back to sleep. I'm 47 and I can't remember doing that my entire life. When I'm on my back I lose the seal and wake up gasping and listening to 'air farts.'

I'm guessing this is because I'm overweight and that when I am face-down my fat fills the seal and when I'm on my back the fat falls victim to gravity, loosening my seal. Any thoughts?

Secondly, I have a ResMed s9 autoset with heated hose/humidifier. I had been getting rainout but adjusted my temp to 69 about 4 days ago - my bedroom is about 70-72 - so the rainout is no longer a problem. I have my humidity level set to 5. I've woken up with cotton mouth for the last 4 days - which confuses me since I raised my humidity level. Even after 3 nights my tank is not empty.

Thirdly - this is a weird one...I'm a really, really heavy sleeper. I'll often wake up in the morning with my mask hanging on my bed-post and my machine turned off, as if I had never put it on before bed. I have to hit the 'info' button to see how long I wore it - anywhere from 1.5 - 5.0 hours. Any solution short of duct tape?

Any suggestions? I'm finding this pretty frustrating. Thanks so much!

Resmed s9 machine with heated hose/humidifier
Liberty Mirage size sm face mask

I think you answered your own question - when you sleep on your back, your mask seal is lost and you get dryness from mouth breathing (the humidifier can only compensate so much), so I suggest you look up Padacheek.com and get some liners which help a lot with bad seals... however you might also look into things like foam wedges to put behind you to keep you off your back.