Cant apply texture to single face

[biggest_kid]

Hey guys got a problem with the face editor. Ive made a small building, and hollowed it out so players can walk around inside it. Ive applied a wood texture to the walls and a different wood texture to the roof.

However, i want to add an iron texture to the inside walls. But whenever i open up the "Texture Application" tool and select a single face, it wont apply any textures to that face. No matter what texture i choose or what face i choose it will not apply. I hit the apply button and nothing happens. Does anyone know why this happens and how i can fix it?

Thanks,
big