Smackie asks a lot of good questions. It could be a lot of things. But in general, a lot of times when someone doesn't climax, male or female, it is because certain nerve clusters aren't getting stimulated enough. I would ask your doctor about it as well, just in case there are any physical issues.
However, the climax is not supposed to be the be-all and end-all of love making. It is the entire act, the entire experience, the intimacy which matters most.
Good luck.
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