Your cropped screengrabs don't reveal much about your comp, so here's a couple of guesses.
Are you using any lighting in the 3D scene? Its possible that, if no lighting is hitting the background layer, you're seeing the layer in total darkness when you make it 3D. Try adding an Ambient light to the scene.
Where is the 3D layer positioned in 3D space relative to the camera?
Are you using any blending modes on the pink layer?