The transmission gears should outlast the life of the vehicle, but abuse can easily damage them.  Extreme damage would prevent shifting into a particular gear.  Transmission gears can chip and damage teeth, which will hinder proper engagement  Be aware that bad synchros can also hinder proper shifting, but either transmission components will require a transmission rebuild.  If the shifting problems are isolated to a particular gear, then it's most likely the synchro or the gear.
FC3S Pro:  Problems, Components - Transmission, Synchros

If you're experiencing problems shifting into all gears, try swapping the transmission fluid.
FC3S Pro:  Problems, Components - Transmission Fluid

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