Not an uncommon problem, they are very easy to cross thread.
I would suggest you unscrew it fully and remove.
Check the threads on adjuster and the thread in the grinder, clean both out thoroughly using a very stiff tooth brush or a brass bristled suede brush. Use a very large needle or pointed awl to ensure the root of the threads
Refit adjuster and check to see it moves all the way down.
Remove adjuster and treat both threads with dry PTFE spray, allow to dry then refit .