When I was dealing with a little hump in my Beseler 16x20 easel, I used a wood mallet and also a steel hammer with a block of wood. I also stood and jumped on it. I couldn't really make any change in it.

This may not be relevant here, but it is something to think about if you still are having trouble. My little off-center hump in the easel was causing a buckle in the paper. No problem in itself, but I eventually traced a 'blurryness' problem to the fact that the buckle in the paper was shifting and settling out after I closed the easel. Thus causing a problem when closing the easel and hitting the expose button in rapid sequence.