Repair shower surround?

I have a three piece shower surround in my bathroom. The wall behind is green drywall. There are two spots right behind the built in shelves that have come loose from the wall and therefore cause the shower wall to bow out. Since this is right where the shelf is we are now unable to use the shelf due to the lack of stability. The two spots are located on the side panel without the faucet and the large back panel. I am trying to find the best way to fix this without having to damage the surrounding wall face, or the surround.