Always worth checking out the "external" part of the selector mechanism for any malfunction first, although a fault there will usually affect all gears rather than specific ones, still it's always worth a try just in case. Remove everything relating to the gear shift mechanism and just turn the selector drum manually, if the fault is still there then there's nothing else to blame, you'll have to go in.
In any case, still be thankful you have a 2-stroke. Valves, cam chains, oil pumps, etc, etc, all to deal with just to get at the gearbox.
