The transmission synchonizers help transition in between shifts.  The synchros are made of brass and wear down under normal use.  Very high mileage transmission will typically wear the synchros down to the point where they are not effective.  Abused transmissions will typically damage the synchros also.  To replace the synchros will require a transmission teardown and rebuild.

It's is especially important to use transmission fluid that is safe for the brass synchros.
FC3S Pro:  Problems, Components - Transmission Fluid

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