Mine did this 3 yrs ago, then again last week.
Tried to lubricate the shaft of the selector turret. Didn't work. Even light oil won't run in.
Carb cleaner on selector turret shaft! Bingo. Ran 3 years without issue again.
This time gear lever started to get stiff going sideways. Straight to shed. Quick squirt of carb cleaner and its away again no problem.
Hope that helps.
Raymond.
November 2020