I don't know but I think it is because you have enclosed your game is a cube and because the sun light probably comes from outside maybe the program is treating the cube as solid and do not letting light pass. Try to run it withouth the hollow cube. If still, add some light and make sure all the textures are ok, and the function correctly. One time I got something similar and added some lights and I finally saw inside my hollow cube, using the predifined texture. If it doesn't work I am sorry but I am only begginning with the program. I hope it helps.