The deck height knob must only be adjusted when the mower deck is raised all the way UP.

You raise the deck, adjust the knob as desired, and then lower the deck. Then you adjust the anti-scalp wheels if your deck has them.

With the serial number of your GT245 you can choose the correct operator's manual and view it here... click here

I bought the machine new and have never had a problem with the height adjustment until recently; have always adjusted the deck when it was raised up. I can't figure out exactly what the knob is connected to; it either cuts about 3.5" when fully up, or about 1.5 inches when lowered; nothing in between is available....

When you remove the mowing deck you should be able to crawk under and look up to see the backside of the knob mechanism.

You might find the answer in the parts breakdown. If not, then the answer is in the technical manual or at the JD dealer.

You defiantly have a linkage issue...remove the deck using the owners manual procedure...just drop it and reinstall...that should fix the problem or identify what the problem is.