On one of the electrics in my school there are 3 scoops. One of which is dramatically dimmer than all of the others. I was just wanting to see peoples' opinions on where the problem may lie. Just to rule out the possibility, its not a problem with the scoop.
Bad/old lamp. Resistance at a plug, high resistance connection somewhere between fixture and dimmer, bad dimmer, dimmer set with a lower trim, dimmer soft patched at a lower setting, smaller wattage lamp in the fixture, DMX issue, fader issue, light board issue, bad lamp base, bad wiring to the lamp base, bad cable or any connection there of, too low a wiregauge feeding it. How's that for a starting punch list. Is the light bulb screwed in all the way?
You say the fixture is not the problem. I assume that you started by swapping fixtures and the one that replaced it had the same problem. Next most easy is to swap the patch between a dim and bright scoop assuming they are on differing dimmers. This will tell you if by plugging the fixture into a known good dimmer if the problem is before or after the dimmer. If before the dimmer or at the dimmer, that's different. If after the dimmer, it's a cable or connection problem.
The only thing you can do without supervision at this point is to replace cables feeding the light. Given it's after the dimmer, it should solve the problem and you know you probably have a cable with a loose connection, broken wires, or one of too small a wiregauge for the distance traveled. If it did not fix the problem, you know it's actually the fixture which you will have already crossed out by swapping fixtures, or between the extension cord and dimmer. That's easy to fix frequently but not something you school should be letting you fix without direct and compitent supervision. Same with doing the dimmer to light board problems. Break something without being supervised or get hurt and it's more trouble than it's worth in doing it on your own.