With only 21% of employees completely engaged in their work, according to a recent Achievers report, it’s no surprise that many workers end up leaving their jobs. Disengaged employees are a major threat to business success, the report noted, resulting in low productivity, high turnover rates, and decreased profits.

SEE: 10 ways to communicate more effectively with customers and co-workers (free PDF) (TechRepublic)

While managers are often to blame for an employee leaving, responsibility also falls on the employee to voice their complaints, according to Caroline Stokes, executive headhunter at talent acquisition firm FORWARD.

“People usually quit or leave their jobs because they have reached the absolute pinnacle of frustration,” said Stokes. “They may not show it. They may not display it. They may have great emotional intelligence, but they may not be able to say, ‘Oh look, I’m really frustrated.’ Instead, their work may have suddenly deteriorated.”

Transparency is key for a healthy workplace, Stokes added, for all parties involved. To create an enjoyable environment for employees, managers must first recognize why they want to leave in the first place.

Here are the top four reasons employees end up quitting their jobs, and what managers can do to prevent turnover.

1. Unequal opportunities

Women still earn on average $0.79 for every $1 men earn, indicating the gender pay gap is still going strong, even for remote workers. Unequal pay is also one of the most common reasons employees leave, according to Dave Denaro, vice president of Keystone Associates.

Not being paid equally makes an employee feel vastly unappreciated said Maria Colacurcio, CEO of Syndio. This feeling of unappreciation doesn’t stop at pay, but continues when it comes to advancement opportunities within the company, she added.

Employees want to feel like they have the opportunity to move forward in the company and to get the promotions they deserve, Colacurcio said. However, supervisors and managers don’t often make those opportunities known, leaving employees to feel stuck and stagnant. This can all change with transparency, Colacurcio added.

“Employers must start being more transparent about what they’re doing to address pay equity,” she said. “How are companies ensuring that they’re promoting fairly and that they’re giving everyone equal opportunity for those promotions?”

This conversation can and should even be opened by the employee, Colacurcio said: “For example, say ‘John got promoted over me for this role. I would like to know, what were the reasons you chose him over me? Because I would like to document these reasons as things that I can work on for next time.’ You’re opening up the conversation, and you’re documenting skills that you need to move to the next level, which is what a company should be doing for you anyway,” she said.

2. Disconnected

Echoing the findings of the Achievers report, many employees feel disconnected from both their coworkers and working environment, said Stokes.

“There’s disconnect with the people that they’re working with,” Stokes said. “There’s a disconnect with division. There’s a disconnect with the management. There’s a disconnect with their leader, and when you have that giant disconnect, it’s very, very hard to get that back on track.”

This issue can also be solved with communication, said Colacurcio. All grievances in the workplace can really be solved with two steps: Communication and action. Employees must communicate when they have an issue, but managers must take that talk and put it into action, Colacurcio added.

3. Routine and lack of challenge

At some point, most jobs become a routine; however, too much of a routine can cause employees to look elsewhere. “Many people leave because they are not being challenged with the work that they need to do, and they’re not learning anything new,” said Denaro.

This point goes hand in hand with the previous: If an employee is bored or feeling unchallenged, they must communicate that dissatisfaction to their manager, or else a disconnect will occur. Without communicating their feelings, employees will remain unhappy, and managers will remain oblivious, Denaro added.

4. Lack of recognition

Another factor that causes employees to quit is a lack of recognition, said Denaro. “Or, they’re not being recognized in the way that they want. Some people don’t like their name mentioned in a big group of 200 people, and some people do like that,” he said.

Employers must find appropriate ways to recognize their employees regularly, and this can often be done via one-on-one meetings, said Stokes. “All it takes, every single quarter, is to sit down for maybe an hour and a half, and to have a decent, meaningful, one-to-one,” she noted.

However, managers must make those one-on-one meetings safe and welcoming. “If people don’t feel like they’re in a trusted environment, where they can speak openly, transparently without fear of being fired on the spot, then it’s not going to happen,” Stokes said

For advice on how to conduct effective one-on-one meetings, check out this TechRepublic article.