It’s hard. At least for me, this is the hardest classes of employees to manager—-it’s much easier to get rid of somebody who is clearly failing or reward and further develop a high performer.
When I say uneven, I’m thinking of somebody who either: (a) is excellent at certain key aspects of their jobs but clearly below expectations in others to the point that it’s an issue, or (b) somebody who performs well but doesn’t always put in enough effort. In both cases, it’s subjective and difficult to determine that it’s to the point where intervention is required, because that’s usually an uncomfortable conversation. Often, you’re the first manager ever telling the person that they need to improve—-even if you’re not the first person to think there’s an issue. Typically, the other person will think you’re an asshole. As another commenter said, they may recognize later on that you were right but normally in the near term, you’ll have strained the relationship. (That’s why managers avoid those conversations. Sometimes it’s cowardice, sometimes it’s deciding that the juice isn’t worth the squeeze today.)
As for how to work on improvement, that always has to be case-by-case considering the issue and the personality. Sometimes the “stop doing that” conversation is enough. Sometimes coaching or mentoring or development experiences can help. If those don’t work, can you shift responsibilities around or find a different role that is a better fit?
Normally you try to gently do those things anyway before you have the direct conversation, and then hope the direct conversation provides some motivation/insight to take it more seriously.
If you’re on the receiving end of that conversation, I guess my advice would be to try to critically self-reflect and see if you can understand where your manager is coming from. If you can, then you’ve both got a common place to start from towards improvement. If you can’t, or you think they’re just being unreasonable about expectations, then it is probably time to move on or hope they move on quickly.