Have you gone back for another sleep study since you began on the c-pap? You may need to have the volume readjusted. My daughter was on a c-pap for awhile when she was younger and had to have her volume changed a few times. Also, there could still be another underlying sleep disorder other than sleep apnea. Once my daughter's apnea was under control, she was still having breakthrough exhaustion, falling asleep during classes, lunch and recess, despite 10 hours of sleep a night. Since I have Narcolepsy, that's where my thoughts for her were headed, so I brought her in again for yet another study. Although she has not yet been diagnosed with Narcolepsy, she has been diagnosed with Idiopathic Hypersomnia. All that means is, she is excessively tired, like a Narcolpetic, but the docs have no explanation as to why.
It took 20 years before I was diagnosed with Narcolepsy, mainly because sleep studies weren't on the forefront of medicine as they are now. Once I was diagnosed, it took several years to find the right medications that actually worked for me. Continue to talk with your doc about what is and isn't working for you, whether you get relief or not. Research medications to see if there is something new on the market. Above all, if you don't feel like your doc is listening, responding or understanding don't be afraid to get a second opinion. I wanted to try a new medication that came out a few years ago that was getting a lot of positive reviews from other Narcoleptics, but my doc always took the more conservative approach instead. When I was at risk for losing my job, I got another opinion. The new doc tried this medication and I haven't felt this good in 30 years! I loved my former doc, but he wasn't working for me anymore. You have to be your own advocate, sometimes.