She jokes around. She makes subtle flirts that you don't think mean anything. You can never be sure though, and that's why you don't stop trying. But on the other hand, you don't know enough about her to make a move.
And every time, without fail, when you decide to make a move, she's not there anymore. You take a look at what happened, and she moved on. She's with someone else.
Once you realize all of this, the next time that it happens you might know better, and it might turn out better for you. But probably not...