Skybox help

[Assassinator]

I am creating a map that is an underground maze. However, there is one area that is in the open air. I have been trying to add light to it, but somehow, skybox's and even light_environments aren't working. I have looked at many different tutorials, none of which have sorted my problem. Can someone please help me?

Cheers

 

[Wind422]

Is there a leak? (Random guess.)