the servo saver, allows the servo to turn without turning the wheels when the wheels are stuck against something or when the wheels recieve an impact.

There is a thumb adjustment on it....screw it down and the servo saver is tightened. if its to far up, (unscrewed) then the servo will turn without the movement transfered to the wheels.

Then there is the servo horn. sometimes the splines on either the servo's output or on the inside of the servo horn can become stripped. no matter how hard you tighten the screw that holds on the servo horn, it wont put the power of the servo to the servo horn....thus wheels wont turn.