I think it's all to do with how comfortable she feels with you and whether she wants to cum or not. I think women have much greater control over whether they want to, or are willing to, have an orgasm. I have had many occasions when I am sure that it has happened (only ever through oral and/or gentle use of fingers) but others where I have been unsure whether it has been faked ("You've made me cum you naughty boy" = instant turnoff) or where it definitely hasn't happened.
In the instances where it's been a definite no or I've been uncertain, I'm convinced that the girls were working to their own set schedule of how they wanted the meeting to progress, and for them to orgasm wasn't part of that plan, so they didn't. Nothing wrong with that per se of course, it's their choice and they still worked hard to please me, but it was a bit of a downside for me. Of course it could conceivably be that some women don't find overweight balding men barely the right side of 50 to be sexual dynamite, but that's just ridiculous...isn't it?
All sex workers have different responses, preferences and motivations for doing this work. I choose to return to those who want to enjoy themselves as well as getting me off.