Girl only comes when she's on top, what is it about this position?

I've been seeing girl for a month, and we've had sex maybe 5 times. She's only been able to come when she rides on top, grinding her pelvis against mine. I'm wondering what it is about this position that works so good?

It makes me feel inadequate. Because I usually have no problem making a girl finish, but I just can't seem to do the trick with her. Every other position I work my ass off and do everything I can in order for her to be pleased but to no avail. Then she rides on top for a minut or two and comes.

Does anyone have any tips? Is there something I can try next time around?