This is a story of two dogs. (And it has a point, so stay with me.) Kipper is the name of one dog. He is a miniature dachshund that I got through a rescue organization when he was a year old. My other dog, Packer (Go Green Bay!), is a beautiful and regal white German Shepherd.
You should know that Kipper is crazy. He is a short-legged, log-shaped bundle of neuroses the likes of which you've never seen. He barks when people leave the house, he goes crazy at doorbells (both live and on TV), and he loses his ever-lovin' mind in a moving car. At the park, he picks fights with Rottweilers and at home he snacks from the litter box (the latter I think qualifies as a crime against humanity.)
Now Packer, on the other hand, is ruffled by nothing. I show that dog how to do something once and he's got it. He's a stoic, Rin Tin Tin-ish dream dog. Dependable, protective, and asks nothing in return (mainly because he can't talk, but you know what I mean).
You can guess which one, out of necessity, garners most of my attention. Yep, the one with all the problems; the squeaky wheel (or, in this case, the pip-squeaky wheel). It's an unconscious thing. It's not favoritism, it's just a matter of my attention being taken up by problems that happen to be continuously caused by the same entity.
I'm using a dumb example to make a point. My point is that this phenomenon can also happen in families, where one child needs more attention due to bad behavior or ill health. And it can also happen at work. (Note: I'm not comparing your staff members to animals, I'm merely discussing a psychological phenomenon.)
If you're a manager dealing with a poor performer who you are trying to get on the right track, you can easily start to take your good employees for granted. The good news is that your good employees have their own personal standards and probably don't need feedback to keep doing their jobs well. The bad news is that's no excuse. It's still your job to nurture and encourage good traits like productivity and dependability, just as much as it's your job to correct the bad ones. At some point, and better sooner than later, you're going to have to step back and take a look at how the problematic employee is affecting the morale of the rest of the team. If the problems can be fixed, then by all means fix them. But take time every now and then to acknowledge the good stuff that's happening, and the good work that allows you to step away and focus on the problems.