The script about sky definition doesn't work in my demo,it's that:
sky Mysky_cube
{
type = <sky02+6.tga>;
flags = cube,visible;
layer = 3;
red = 170;
green =170;
blue = 170;
z = -50;
}
I think it's right,but why doesn't it work?

Do you have a hollowed cube with a texture with the skyflag set around your level?

No.Does it have to do with that?

Just try that...
Just use the default texture with sky flag.