Why does this mask leak so much now?

I have an Ultra Mirage II Nasal CPAP Mask with Headgear. It has worked for me for about 6 months now without a hitch and leaks within acceptable parameters. For some reason, just recently the leaks are out of contol it seems. I just replaced the entire mask with a new Ultra Mirage II Nasal CPAP Mask with Headgear from the doctor and it is doing the same thing. I have noticed that on exhale, air escapes from the sides of the gel piece, whereas it never used to do that. The only thing that is different is that I am about 10lbs lighter. I have been tinkering with the straps and notch settings at the nose bridge. I can't get it to stop. I wish I could tolerate my nose pillow mask. The gel inserted into my nostrils irritates the inside of my nose with a burning sensation after about two nights. Any of you guys have had a sure-fit mask go flatulent on you too?

Try this: Shave before bedtime. Scrub mask sealing surface with washrag. Scrub face where mask touches with washrag before going to bed. Watch the CD again that came with the mask and adjust the forehead piece where the mask seal sets firmly on the nose bridge. Tighten the bottom straps 1/4 inch (1/2 cm) at a time until the mask seals properly. Leave the top straps just barely tight. It is a good mask and should seal properly with a little encouragement!

I just got a Mirage II Nasal Mask and I'm having problems with it too. I can't get it to stop leaking. Even if I get it sealed it begins to leak after I've been asleep for awhile and my husband is shaking me awake to tell me. At times when it does seal, it's the quietest mask I've used, so I hate to give up too soon and go to another mask. I got no directions or video - I looked at the Resmed website's PDF manual and it only shows how to break it down for cleaning - no usage instructions.
There's a little plastic collar inside the mask at the junction of the mask and the swivel. Does this need to be positioned in a certain way? If not, what is the purpose of it?
Thanks in advance for your input - I'm recently back on the wagon (mask) and want to make this work.