A reason you possibly still feel tired on pap therapy is it may not yet be optimized. Sleep doctors don't consider patients to be fully treated until their apnea index is less than 5. And many patients don't feel better until it reaches less than 2.
For further assistance, you might want to visit cpaptalk.com which is a self help group of sleep apnea patients helping each other with various issues. There is no charge to join and it extremely helpful.