For people who are having trouble to turn the elevation screw on the top, it is most likely the set screws under the flap have backed out to push the flap upwards. As a result, the flap cannot be pushed downwards. That makes the elevation screw extremely hard to turn. You just have to use brute force to turn it - make sure you turn it up not down, because the flap won't go down further with the set screws pushing from below.