When you flip the binding up to make the fold , is there a true 45 degree angle? If I get careless, and don't make the fold a true 45 degrees, I get what you get. Also the fold along the top edge needs to be flush with the quilt. I sometimes make sure that the top fold is even a few threads above, which gives a really sharp corner.
Scroll down to where Holly is talking about the corners and you will see.
http://fingerlakesart.blogspot.com/2...-tutorial.html
Good luck