If it's purely a business transaction in her eyes and she's just tolerating it for your hour, or whatever, it's no surprise that she won't get as aroused as you might like.  That's one difference that separates the ladies that can really get into the appointment and share the pleasure from those that are just waiting for the hour to be up to collect the money and get you out the door.

I dated a lady for several years who swore she had never had a orgasm in herr life. It was the way here body reacted. Was it a mental thing, or a physical thing. I don't know, but she still enjoyed our time together and would instigate bed time play. So who knows. If your relationship is good, then don't worry to much about it, just make her happy, as best you can.