Well I think it has largely been said., but I'll add my 2 cents. I've been all over the place. I'm primarily a bottom, so if I'm with a client, I almost am always hard getting going and sometimes am hard throughout the entire bottoming process, sometimes not.
Really a variety of reasons, but not being turned on certainly isn't one of them. It can be number of clients, just the events of the day, but it could imply a lack of something with the client. That usually doesn't happen. I wouldn't assume if he isn't hard and doesn't climax, he isn't having a good time, however.