Are you using any mod that alters the ambient light of the dungeons? I know of a few "offenders" and I use one myself. Vanilla Oblivion dungeons are quite bright and you barely need a torch to navigate them, at least I don't. I'm asking this because, since your Brightness option is locked, that might be a problem not related to any mods. Also, does everything else look appropriately bright (or dark)? If so, that's a strange issue.
If you are using mods, just deactivate them or, if you don't mind a bit of nibbling in the CS, turn the ambient light up a few notches (quite a hefty work, I know...).
EDIT: I've found a mod that may help you: Brighter Caves. Haven't tested it myself but it must do what it says, which is make most caves brighter. This saves you from the hassle of going into the CS and tweaking the ambient light cave by cave. All this provided it's just an in-game, dungeon problem and not something more serious, so to speak.