Joe,
I had this same problem, last year, and the good folks on the Grumble gave me the following winning advise;
The first two are answered here. Take the little screws out and insert into the oposite side.
Make sure that FACE is towards the art
If this fails try the following.
Again, take out the little screws. take a small piece of glass and lay the plate on top of the glass. move the plate back-n-forth. Try both sides for wobble. If their is a natural wobble on one side take pliers and bend against the bow slightly and retry. Takes a little time but works for me everytime.
Also, clean-up the edges with sandpaper or better yet your Manual 45 sander (yes)
best