I think my absolute favorite was Harry Kim never getting promoted (even though everyone around him was) but somehow retaining a critical position on the bridge and the command staff. If he's not worthy of promotion, then bring in someone who is.
How is that a contradication??
It's a critical position, which means it should be filled by someone who's trusted and highly capable. You can put a bright, promising ensign there, no problem. If he does well, you keep him there and give him a promotion. If he doesn't do well, you shuffle him to a different department where his skills can be utilized better.
But if such an officer is never promoted from ensign after seven years, it's because he's either not trusted or has proven himself incapable. In neither case would you simply leave him in that critical job.