I have been trying out and was turning the knob that adjusts the headlight up and down, and came to a point where it is now too low and would not go back up. The knob that i am talking about is at the back of the headlight housing, on top of the snap-on cover. Am I doing it wrong? The knob tightens-up but the beam would not adjust up any further. my car is a 91 560SEL

thx