As I understand the 110tlb was sold through the green guys not the yellow guys.
Are you positive the shaft is turning? Sounds like the knob may be slipping on the shaft. It should screw in and bottom out and resist turning with the knob. Then you should be able to back it out and vary the rate of drop with a turn or two.
FWIW the 110 is based off of a 4600 compact tractor (green)[/QUOTE
In this neck of the woods it was sold by both yellow and green dealers which was a plus for parts. My green dealer has been recently unable to get certain parts and attachments for the 110's but the yellow guys still can. I agree with Steve, ensure the knob is actually turning the shaft.