Go in and make sure all of the wires attached to the "dimmer" one are properly secured. If that doesn't fix it, try to switch the problem bulb with a new bulb, or the bulb on the other side and see what happens. If you are still having problems, post again and let us know.
One more thing...
If you get out of your car and squat in front of it with the lights on, is it the main headlight that is dimmer, or is the lack of brightness on one side being caused from a parking light or such being out? Let us know.