Chemistry makes for the best encounters but it's not something you can expect when you are seeing someone for the first time. Chemistry can't just be ordered up, but as others have mentioned, it can develop over time.
If there is no connection but the lady is a good professional, which includes being pleasant and agreeable, that's usually enuf for me and I would probably repeat.
To me chemistry just defines the interactions of people who genuinely like and naturally connect with each other. it is present (or not) in all sorts of ordinary situations we find ourselves in all the time. I was chatting with a bank teller the other day. Lovely lady. I thought we had great chemistry. Our interaction was clearly more than transactional. It was personal.
Good chemistry by itself is however not a guarantee of success. I see an MP with whom the chemistry is very good. It's clear we like each other as people. Her reactions are genuine but our relationship is unsuitable as a platform for good sex. This is because these days I like a lady who is a little assertive. I need that to get in gear, then I'm good to go. This lady, like me, is relatively passive. What she needs, and gets, are (younger) clients who take the initiative and push the envelope. She then decides how far she is prepared to go, depending upon how she feels about the client. With some clients that results in a full and complete menu. With others not so much. Personally I never even make it to the decision stage lol.
I see her only for massage these days. I do that less for the massage and more simply because I like the person and enjoy seeing her. She is after all a beautiful young woman and to all appearances a good person. I visit one of her co-workers when I want something else, a more experienced, much older less attractive lady who is a real professional and who intuitively knows how best to deal with me. This seems to puzzle the younger one. If she were more experienced I think she would be able to figure it out. Whether if she did there would be a change in her service with me I don't know. I tend to doubt it.
Or maybe some would just categorize this situation as exhibiting a lack of chemistry.