No Floors

[Cyphermur9t]

I really don't understand and it's starting to get infuriating. I created a simple hallway using the default textures, but a section of flooring just refuses to appear in-game. I've tried recreating it, changing textures, saving as a different map file, moving the entire map over, NOTHING works! the thing is, the rest of the games floors work just fine. It's so stupid!

THIS