It could be the ball bearings under the u-turn turning mechanism right on top of the fork?
I have had this problem….water gets in and the bearings get a bit rusty then don’t turn. Just remove the top cap and the bearing will be underneath (BUT BE VERY CAREFUL not to lose the bearings as you open up the top cap) and I would suggest doing it over a big white blanket or something similar in case they do drop out.
Make sure you also have a magnet handy in case the bearings slip down the bigger holes next to the ones they are sat on top of (you’ll know what I mean when you take the top cap off).
Once open, take out the bearings (make a note of where they were seated as there is not a bearing for every hole), clean the whole thing up, regrease and put back together – its an easy job and it will sort things out for you!