Does it not move at all or just not appear to change anything? If it doesnt move at all then there is an issue. If it just appears to not do anything then...
What race are you? I have my gamma turned all the way up (my monitor is pretty old and is extremely dark). Even with full gamma on CRT and in game, my human druid cannot see in some tunnels at all (I have to navigate using the map), however, my gnome enchanter has no problems in the same tunnels - it is the racial trait (and my human is carrying the newbie lattern that humans start with).