It could be a lack of connecting with her....I have that issue as a girl, which wasn't so bad when I was younger. I used to be able to just be flirty and cute (but still awkward, drinking a little too much to feel comfortable with eye contact and still talking too much), and the guy would look past other issues for a while because he was attracted to me. I'm not so young anymore, and I'm getting to the end of my cute stage in life. ; ) but for me, I really think it has been my inability to connect with others. The reciprocity is my main social obstacle I feel.
You could always ask one of them why they lost interest. Might be weird if a lot of time has passed, so maybe if it happens again..?.