Hi there,

This should be a case of adjusting the selector on to the protruding shaft on the gearbox.
The 13mm tightening nut does have a tendancy to loosen causing the rod to not push the selector shaft into gear correctly.
Another way of lining up the selector rod onto the gearbox shaft is by looking at the shinyness on the gearbox shaft where the rod used to be. The other piece not covered will be dirty and the section that was covered will be bright and golden coloured.
I have 3 spare selectors here should you need one.