maybe clean off the grease and see if the threads look damaged. Or the threaded part could be bent. I am dealing with a similar issue, where I actually removed mine and put in a vise to unthread completely so I could replace the thrust bearing in the gearbox. It was very difficult to do and I must have actually damaged the threads enough in this process that a couple months after putting it back together I actually had the threads pull out of the bottom part and the right side of my mower deck fell to the ground then when I lowered it I bent what was left of the threaded portion.
