I agree with the too much fabric diagnosis. When you attach the binding and get to the corner, you make a 45 degree fold and then fold a 90 degree back onto itself. When you make this last fold, you have to make sure the fold doesn't extend past the edge of the quilt. It needs to line up exactly on the edge. If you go over the edge, you wind up with too much fabric to make a nice miter. If your fold is short, you won't have enough fabric to make the miter. As mentioned above, you would need to take quite a bit of the binding off to fix it, but it's not that bad.