You need to pay for the session, unless it is a case of bait or switch or she is intoxicated or something bad in that sense
or if she was very late, like more than 20min or so
then if you feel no chemistry or attraction, what I would suggest is that you tell her you are having a bad day, ask her if it is ok to start with (receiving) a massage, if half way you are still not feeling like getting a BJ or HJ, just tell her you are tired or stressed at work, whatever, get dressed, thank her and leave
simply don't repeat, having bad sessions is something that happen from time to time, hopefully not too often if you do your research right