Most likely the selector at the base of the lever is stiff through lack of use.
Very common fault.
you can try changing the fluid but I doubt much it'll be that.
Quite easy to check if it is low and top it up.
To sort it you take the centre console out, drill through the pop rivets, remove the unit on top of the gearbox and take it out then give it a proper freeing off and lube it.
However since it does move, it might be enough to give it a good working to and fro, left to right, from time to time.
By far the biggest pain in the whole job is putting new pop rivets in where you took them out from, the rest is pretty easy.
Of course others use other forms of fastners.
Technique for use is in the handbook.
Doing it on the move needs practice after all there is no syncro on it!
Best of luck.
