take the left cover off and look at the selector arm and claw. maybe the bush in the cover is now worn? the later bikes won't allow the backwards/forwards plate to slide nicely across the up/down baseplate. the 'claw' gets a smashed nose like a boxer. remove the blunt material and chamfer the edges.
also, remove the 'star' and turn the selector drum clockwise as much as possible by hand, keep spinning the rear wheel while in gear and keep turning the selctor clockwise to the maximum. then go anti-clockwise just a little-little bit. STOP.
this should be the 'special place' on the star for neutral for the 'wheel' to sit in AND you should have neutral too!
sometimes this 'star' is one place out etc etc.
Taffy