I had the same problem.. After about 18 months of use, the knob got harder and harder to turn until it was unusable without a pipe wrench. :mad: As Jim Elder mentioned above I think a little lube is the cure. As the warranty was done and the unit was unusable I tried the following:
Step 1 - Pull the knob off
Step 2 - Set the unit on its back
Step 3 - Apply a few drops of lube directly onto the post ( I used WD 40 )
Step 4 - Wait for 5 minutes
Step 5 - Reassemble and work the switch for a bit ( with the pipe wrench if needed )
Worked for me, knob turns like new, but no guarantee this won't cause problems in the future.
I did this about 6 weeks ago and use the unit almost daily with no problems yet.