One thing a lot of guys do not understand about "chemistry": it is a complex thing. Maybe you are doing everything right, but her parents are going through a divorce and she stressed out by the exams right now so she is only receptive to 50/60% of your whole energy. Guys, unless you Brad Pitt and go to her telling her "hey, baby, I am Brad, I fell in love at first sight and I will solve all your problem. Your exams? No worries, private teachers, the best. Parents divorcing? No probs, marriage counselors, the best. You up for a drink now, beautiful", there are times when it's not going to work.
The point being, it does not depend on you only. There is another person involved, hello...Other than this, girls are highly, highly attracted to certain guys for some reason noone knows. It doesn't only have to do with how good you are at making her wet, sometimes she wet already because she sees something in you that we cannot explain, she also cannot explain.