Did you make all 4 walls the ~LIGHT3A ???
1) If so, try making just the ceiling that texture.
2) You'll need to edit the lights.rad file (text editor) and change the brightness of that texture lighting.
NOTE :: When you modify this, it will make ALL the textures you compile AFTER this modification all have those texture lighting. It's not only for that current map you're compiling.
So whenever you start a new map, it is suggested that you revert back to the original lights.rad file so your next projects don't use those same texture lightings.