glsl & linking problem

I’m working on a 2player shooter for me and my cousin to play. I created the characters and weapons and gave them materials. I enabled glsl materials and played the game. The glsl materials looked awesome.
I then linked the groups for my players and weapons to a level I created. When I played the level with glsl enabled the linked objects were grey. The level also flashed grey a couple of times. Is there any way to fix this?