[notact]

Mine is concave too, I think this is a common problem. I did the same as you, I raised the front & back center hex screws slightly (as seen from the front of the table). With the phillips screws in the corners tightened down, the plate should not rock - this effectively bends the plate's concave center into flat, although doing this while maintaining the plate's flushness with the surrounding table is like a game of whack-a-mole.