Here are the top five reasons good employees 'go south', how to spot them, and what to do about it. Note that all of these boil down to poor manager-employee relationships, as the line manager is the person ultimately responsible for managing top employees:

1. Inconsistent / frequently changing priorities.

WHY IT'S A PROBLEM:

Nothing irritates a top performer more than 'ditch to ditch' or fad-based management.

HOW TO SPOT IT:

Employees 'hunkering down' every time a new 'initiative' is introduced. Glazing over at 'strategy' meetings.

WHAT TO DO ABOUT IT:

Set a short- medium- and long-term strategy and stick to them for a reasonable period without being distracted by the newest new thing. See John McCain's campaign strategy for examples of how *not* to do this.

2. Condoning mediocrity.

WHY IT'S A PROBLEM:

The #1 reason high performers leave organizations in which they are otherwise happy is because of the tolerance of mediocrity.

HOW TO SPOT IT:

Disdain and distance between top performers and others who are not pulling their weight. Dissatisfaction with rewards (compensation, bonuses, awards, etc) given to others.

WHAT TO DO ABOUT IT:

Set high goals for the entire organization, and build in both rewards (for success) and consequences (for failure). Apply both consistently and fairly.

3. Round peg / square hole syndrome.

WHY IT'S A PROBLEM:

High performers like to do what they're good at - not used as a a stop gap in some other way. They view themselves as Ferraris, and get frustrated if they think they are being used as a golf cart.

HOW TO SPOT IT:

Disengagement from their allocated tasks and responsibilities. Lack of follow-up and accountability. General mopiness.

WHAT TO DO ABOUT IT:

Review (with them) what you want this person to do. Freshen up job descriptions and re-orientate top performers to tasks that only they can do.

4. Underutilization.

WHY IT'S A PROBLEM:

Same as above - when you're a Ferrari (or think you are) you don't want to spend your time idling at the curb.

HOW TO SPOT IT:

Freelancing in areas that aren't their responsibility. Getting under everyone's feet. Going rogue.

WHAT TO DO ABOUT IT:

Have the employee produce a list of what they could/should be doing to occupy free time. Review and agree on utilization. Look at your own delegation skills - if you have an underutilized top performer, it's a sure sign you're a micro-manager who has problems delegating.

5. Playing favorites.

WHY IT'S A PROBLEM:

Top performers not only believe in a meritocracy - it's their air and water. Start playing favorites and bypassing people despite their results, and your top performers will be out of there before you can say 'Holy second cousin'.

HOW TO SPOT IT:

Your sister Sarah's son Jimmy seems much happier than your best sales person.

WHAT TO DO ABOUT IT:

If you need to be told, you shouldn't be managing people.