Ubiquity,
A photo of the table might help, especially from the underside. If you have access to the bottom of the playfield, one thing you can do is to glue blocks ('nailers' in carpenter terms) at each end under neath the goals and one in the middle of the playfield. Once dry, you can then take a 2"-4" running lengthwise and while holding the playfield flat (downward pressure at the center, from your description) screw the 2"-4" into the nailers. If you can't get to the bottom of the playfield you likely would need help from the manufacturer to have yours replaced.
Good luck........................................Tyler