Thank you for contacting CPAP SupplyUSA.COM. Youmay have the right size mask, and even though you are wearing a full face mask, you may be opening your mouth so wide at night that the mask slips out of position. Sometimes CPAP users may have to wear a chin strap also to help keep the mouth in position. Also take a look at the Fischer and Paykel 431 .This is a full face mask that comes with three cushions (S,M,L).The cushion has a very comfortable chin support that may help in your situation.
Try a chin strap first. This is less expensive and is simple.
Hope this helps
Debbie Shirley,RRT