Mechanical retracts can be trying at best. It pretty much all comes down to geometry. Having a disc large enough and positioning the linkage rods at the correct distance away from center of the servo is the key so the travel is just right to acomplish full travel up and down. I assume you have a retract servo with 180 degress of travel. Did your instructions have any diagrams? Sometimes you need to bent a dog leg in the wire at the servo end to clear the opposite side wire. I will try to find some pics demonstrating this or maybe someone else will post some.