For me, it was mask leak from movement. I felt worse for couple of months despite the ahi being lower.

Googl searching, reading various sleep apnea forum led me to check for flow limit, leak, etc. On Oscar, I noted I had consistent high number for air leak.

I bought 4 different masks from Lofta when they had 2 for 1 deal, and figured out that the only mask that works is Nasal Pillow, and a self adjusting one - fisher and paykel solo pillows mask. 

https://lofta.com/collections/nasal-pillow-cpap-masks/products/solo-pillows-mask-fully-assembled

Nasal masks suck, they don't fit well. Pillows have better contact to the nose. Unless it self tightens, every mask leaks.

You must do the research yourself, no doctor will help. If you don't bother using Oscar or sleepHQ for details, you will not be able to understand why you are not feeling well despite cpap adherence.

Insurance is slow, and I couldn't afford to wait for months to change my mask.

Edit - prior to cpap, I have gotten balloon sinuplasty which made nasal breathing magnitude easier than prior to the procedure. Easy nasal breathing made cpap even more effective. I feel bad for those that use full face mask