That's an aluminum frame, isn't it? There are limits to what the frame threads can take. (I might think about drilling out those holes and using longer high quality bolts and huts + lockwashers on the wheel side if it were my bike and I really wanted to ride fixed.)

You have probably already thought of this, but you have taken de-geaser (or strong detergent) and cleaned the surfaces of both the frame and the dropout mating surfaces I trust.

Ben