Corporate executives might know their revenue and performance metrics inside-out, but they don't always have their finger on the pulse of their workforce.

They conduct engagement surveys and reviews, but those tactics won't necessarily reveal the root causes of employee malaise. Workers may hold back on sharing the unvarnished truth, and direct managers may not have complete clarity or objectivity either.

Perhaps that's why we're in the midst of an employee engagement crisis. According to Gallup, only one in three employees can be described as "engaged," costing employers $450 to $550 billion in lost productivity. This clearly won't be solved with donuts and ping-pong tables.

My company recently commissioned a survey of U.S. employees to see how they really feel about their jobs. Here are a few of the more alarming findings:

  • Nearly half (43 percent) of U.S. full-time office workers confess they're bored or disengaged at work.
  • "Boredom at work" is a factor for nearly two-thirds (61%) of employees who say they're "likely to change jobs in the next 3-6 months."
  • Millennials, now the largest generation in the U.S., are nearly twice as likely to be bored at work than Baby Boomers.

You need to keep an eye on bored workers. They might have tremendous potential, but something is holding them back from putting forth a full effort. Often it's that they don't feel challenged and rewarded and, hence, would rather work where they can stretch and grow.

That insidious attitude can spread if left unchecked. Our survey also found that disengaged workers are 2.5 times more likely to quit. Half of those surveyed said they often hear their colleagues express feelings of boredom; those colleagues are like canaries in a coal mine.

When the people around them are disengaged, ambitious and motivated individuals may conclude it's not the right environment for reaching their goals either. This hits the bottom line hard; hiring and onboarding a new employee can cost 1.5 to 3 times their salary.

I'd like to think every person begins their employment at a company full of optimism and a vision for their future. So where do employers go wrong? How does that initial engagement fade and what can be done to sustain it?

Here's what I've seen work at my company and others:

1. Relax your rules

Millennials, in particular, resent and resist rules that don't make sense to them. For example, many companies still expect butts in seats eight hours a day, while showing less regard for whether people use that time efficiently.

Today's employees want flexibility in when and where they work. To retain the best and brightest, companies need to learn to be adaptable, not the other way around.

2. Recruit from within

Employees are humans, not static resources. Their interests and goals will likely change over time, but your business needs will change too. If you can find where those two lines intersect, you can close your own skills gap with the talent you already have.

In the 2016 Udemy Workplace Boredom Study, 44 percent of people said "unchallenging work that doesn't use my education/background" contributed to their disengagement. Rather than let someone walk out the door, smart companies find creative ways to reengage these people, such as with rotational programs and cross-training.

3. Offer learning opportunities

Also from our survey, an overwhelming 80 percent of employees agreed that "being given opportunities to learn new skills at work would make me more interested and engaged in my job."

To be clear, this is about letting people control their own development so they become more valuable overall contributors, not limiting them to learning skills directly related to a current role or areas for improvement.

Millennials are leading the way here too, demanding learning and development that's stimulating, not required drudgery. Smart companies use learning to motivate, not to point out deficiencies.

4. Make reviews a conversation

At Udemy, we made this shift about a year ago, following the example of many top-performing companies. Rather than conduct traditional performance reviews, which don't have much impact, we encourage managers and their reports to maintain an open, ongoing dialog where it's safe to talk about roadblocks and concerns.

That's also where we want employees to be transparent about their short- and long-term career goals and how we can help them get there. If a high-potential employee is bored or dissatisfied in their current role, we want to know about it right away so we can work together to find a better fit.

The average job tenure is shrinking, especially among younger workers, and companies are in a constant battle to secure the best talent. Once you've made the hire, you need to stay engaged with your employees if you want them to stay engaged with you.