Diagnosis
The detent mechanism inside the knob has failed or become disengaged. The knob is no longer making proper contact with the internal switch.
How to Fix It
Unplug the unit. Gently pull the knob straight off the shaft. Inspect the back of the knob and the shaft for any broken plastic teeth or debris. Use a can of compressed air to blow out any dust from the shaft socket. Realign the knob's flat side with the flat side of the shaft and push it back on firmly. Test.
If That Didn't Work
The internal switch assembly is likely damaged. The knob spins a potentiometer; if the knob itself is intact, the fault is internal. This requires a repair technician.
Heads Up
Do not force the knob or use tools to remove it. This is a press-fit component. Forcing it can shear the plastic shaft.