Are you seeing a doctor who specializes in sleep medicine as followup appointments?
Or are you just calling the sleep lab? I do hope you schedule followup appointments. And I agree with glennaa11, if your sleep doctor isn't helping, find another, if possible.
As was said, why weren't you told to get a full face mask? Or if you prefer the nasal pillows, then maybe the Resmed Liberty (covers mouth but uses nasal pillows). Or why wasn't a chinstrap suggested? And do they really know why you're mouth breathing? Some people are mouth breathers out of life-long habit. Others are mouth breathers because they have some issues with breathing through their nose, like a deviated septum or chronic allergies -- and for them, restricting the mouth just doesn't work. You might not be asking the doc enough questions, but you should have a doc who knows what to ask, to better help you. As has been suggested, consider getting another sleep doc, if you're not getting answers and help. But you got to be pushy about asking questions and getting help.
It's not your cpap having stopped working for you. It's your medical care professionals preventing you from getting the help you need to get that cpap working for you.