Why Are Composite Deck Boards Curling?

I have a deck that's about 25' square.  Last summer, I installed composite deck boards to it.  By springtime, the ends were curling up, the joints were spreading apart and the boards were shifting.  The semi-hiden fasteners went into 3/4″ plywood. The manufacturer is not helping me with this problem yet.  I don't know what went wrong. Can you help me?

                  Thanks,   Bob

The Money Pit Answer

Composite decking is generally installed on dimensional lumber.  The fact that you've attached it to plywood might mean it's not properly supported, and the composite lumber was potentially never given the opportunity to dry out.  Those two issues together could be causing the curling. 
Unfortunately, I doubt there is a way to correct this short of replacing it altogether.  If it's not a tripping hazard, I'd learn to live with it.