Well I know for a fact that a few times it has been because I hadn't struck when the iron was hot. I was too timid to make my move, and sat around waiting.
Time is a factor. Noone is going to wait around for you for ages. Even if they do like you, they will grow weary of waiting for you.
Of course all of this only applies if there was some sort of mutual attraction to begin with, but claiming that this phenomenon does not exist is false. I have had this confirmed years later by the girls in question. I learned from this that I should not be such a pussy, and that is the lesson everyone should take away.