If you are seeing her on a regular basis, then you should be comfortable enough to talk to her about it. Ask her about her turn-ons, ask her if there is a different way that she likes to have something done, mix up your routine.

It can be hard to keep focus on what your role is in a relationship like this, even harder when you see each other regularly and are comfortable. She may not be a nympho, there may be times she has had her fill or isn't in the mood, or it might all just be an act and she is getting complaint due to your familiarity.  
As much as we like to ignore it, she is getting paid to make you feel like a king. If your going to worry and deal w complications you might as well get a f*** buddy. If you two have a connection you don't think you can replace then definitely try talking to her, otherwise why not find someone who enjoys doing this (or at least acts like they do)

The session is about your pleasure- not hers. Some providers hold back to focus on the client's enjoyment, others fake it if turns on the clients, and a few, very few, get off on their sessions, but not always. You can't expect a provider who is seeing a couple of clients a day to be a cuming machine unless you believe the reviews. It sounds like you need to have a talk with her, but be aware that it may seem "fake" to you because most likely, it is.