I had faced this problem of good subject once.
Two girls who where friends to each other comes to me for some portraits. One was beautiful and other was looking good.
But when I started shooting the other girl who was looking good to me, I was unable to get a satisfying shot. I tried to change the light but still no success, but I didn't make it obvious to the girls that I am not getting the thing I was looking for.
In this process the first girl thought , I didn't give her much time. But later onn after seeing the prints, She was most satisfied and the other girl didn't show up again after comparing the photographs of her beautiful friend.
Still today I think that
1. How I would have worked the scene. I failed and didn't tried much.
2. Better subject results in better images.