A lot of times it's a mental block. She's so worried about not having an orgasm that she just can't focus on the experience and lose herself in the moment. Try to remove any pressure from the situation. Instead of trying to make her have an orgasm, tell her you just want her to enjoy the experience. Take it slow, see what feels good but don't get hung up on making her cum, and don't act dissapointed if she doesn't get off.
Give it time, it can take a long time for her to truly lose her self-consciousness, but once she does she'll have the orgasm of a lifetime
