When your team's star players are performing at a high level, it's easy to assume that everything is fine. However, it's important to keep your finger on the pulse of these office standouts. While they're the least likely to grumble and whine, they may be eyeing other opportunities for a variety of reasons you're not paying attention to. Here's why your best employees might be considering a job elsewhere.

1. They're burned out.

When employees provide great work, they're often rewarded with even more. Treat your most capable staff members as the MVPs they are, and make sure they have support and time off to make up for the long hours they put in. Avoid placing too much expectation on one person's shoulders to the point where they become quietly overloaded. Be sure to rotate everyone on your team, so one person is not carrying the weight of the entire group. Find out what they need to perform at their best and make sure they have it.

2. They don't have good tools.

Things you might consider trivial -- the copier that jams continuously, the outdated computers, the lack of support staff -- all send a message that you are not willing to invest in your team's productivity. Do the best you can with the budget you have to work with and periodically ask your staff if there are issues that need your attention.

3. They feel unappreciated.

If you consider an employee's paycheck as a twice-monthly recognition of a job well done, you need to step up your game. When employees work hard, getting acknowledgment from a supervisor for their efforts can make a huge difference in their morale. This isn't one-size-fits-all; different personalities respond to different forms of appreciation. Find out what is meaningful to your employees, whether it's words of affirmation, public praise in the next staff meeting, a day off, or a cash bonus. Consistently let them know you notice and value their efforts.

4. The environment is uninspiring.

The workspace should be clean, comfortable, and as pleasant as your budget allows. Your employees are spending most of their waking hours at their desks, so it's important to have a place where they feel energized and motivated. Make the area as nice as possible with perks such as creative artwork on the walls, a clean fridge, fresh plants, and whatever else may motivate your employees to be productive.

5. They aren't growing.

Great employees are self-motivated, with an instinctive drive to succeed. That means that they're also looking to develop and progress in their careers. If they don't receive additional challenges, responsibilities, and opportunities within your organization, they will likely look for those things elsewhere.

6. They want to make more money.

When you add up the cost of employee turnover, including recruiting, hiring, and training, it makes sound financial sense to take care of your most valuable employees. Make sure you're offering them a competitive salary, and reward them with bonuses when you can. Don't wait until they ask you for a raise; proactively monitor their compensation package and make sure it grows, along with their value to the company.

7. They don't see a future.

If you want to be more than a stepping-stone in a great worker's career path, find out his or her long-term goals and do what you can to be a part of them. If a promising employee is eyeing a management position, give that employee opportunities to develop into that capacity. Grow talent from within your organization by giving more responsibility over time and offering plenty of ways for employees to advance.

8. They aren't heard.

We all want to feel as if we make a difference. Foster two-way communication by asking employees for their opinions and keeping them informed about the company's achievements and challenges.

9. They are unclear on your vision.

Highly effective people want to know they're playing for a winning team. Make sure your values and goals are crystal-clear to create a well-defined path for everyone in your organization.