Haha I like that, how your confidence sways with the seasons. You sound like a real guy and I'll do my best to help. From what I've read she definitely has shown some interest in you. Who goes out of there way to give someone a back rub? The part where she said she would be back but didn't, don't worry about this too much girls are always on the move when they are out.
Have you tried showing interest in her? If not do it, compliment her than ask a question. For example slightly touch her hair and say: "You're hair is so soft and perfect, what kind of shampoo do you use?" This may sound ridiculous but, every time I used it, it makes the girl giggle. It doesn't have to be that compliment it can pretty much be anything you specifically like about her and compliment her on it.
Don't be a pushover though, you can give compliments to girls and still maintain a dominant state of frame. It feels like she likes you because you're being yourself. That's hot, girls like that. Play with her longer and start building comfort. This is the perfect opportunity for you to "Cube" her. Learn how to cube it's golden. I'm not a fan of routines or canned material or anything of the like however, some are very effective, they are fun, and they work on so many levels such as "The Cube"
Next time you see her say something like "Hey you, what's up. What happened that night you said you would come back to hang out? I waited in that chair for
HOURS. I can't even talk to you anymore

" if you do use this use it playfully or it will definitely come off as needy.
In essence the next time you see her make a genuine effort to build a sexual relationship. Be with her longer make friends with her friends and hang out with them if you have to.
Focus on spending more time with her.