Sometimes the clip that holds the outer jacket of the cable slips over the years. The inner wire is fixed at both ends and you adjust the travel by a metal clamp that grips the outer jacket. It can be popped out and pressed back in using a different, longer or shorter positions to make the adjustment. Make sure the spring metal clip that holds the outer jacket is not spread open too much. If you fully disconnect the end on the blower box, you can then rotate the dial to make sure it is not the knob vs the wire end issue. It could also be in the blower box, but that is a much bigger issue to fix, it is full of vents that rotate and has Freon and coolant inside it.