well, shift forks get bent (usually it's second gear that this happens to) because of slamming the gears. if you slam it into 5th a lot then fork could get bent, but like i said, that's worst case, very unlikely.
i think flushing the tranny oil is def a good idea. you might have a piece of gunk stuck in the drum not letting the fork move into position properly.
i don't think this is symptomatic of damaged shafts or cogs because you didn't say anything about strange bumpiness or vibrations in that gear.