Input voltage and type of 317 used aside, the 120Ohm resistor is too low for such an output voltage (around 50V I assume). When you use the wiper of the pot and place it towads the upper half, current through the 120 Ohm resistor and the pot increases too much. Increase the upper resistor and potentiometer, ensure that the pot will dissipate significantly less power than its rating, 100mW is typical. if you want to reach the upper end of the pot but you dont want to use a high upper resistor, add a resistor in series with the pot. This will limit your adjustment range though.