The organization's online survey of 5,641 employees found that 55% of them, the majority of American workers, left vacation days unused in 2015. Project: Time Off also surveyed 1,184 managers who are company decision makers.

The top five workplace barriers for taking vacation is that 37% of employees don't want to return to a mountain of work, while 30% said no one else can do their job, another 30% said they cannot afford a vacation, 28% said taking time off is harder as you grow in the company and 22% want to show complete dedication.

But Project: Time Off's research found there are deeper reasons why employees don't take vacation.

What's more, additional research found an interesting trend related to why millennials don't go on vacation.

1. Millennial Shaming

Vacation shaming – being made to feel a sense of shame or guilt from co-workers for taking a vacation – has become prevalent in the American workplace, especially among millennials, according to new research from Alamo Rent A Car.

Findings from a 2016 Alamo Family Vacation Survey showed 59% of millennials reported feeling a sense of shame for taking or planning a vacation compared to 41% of those 35 or older.

Interestingly enough, millennials aren't just more likely to feel vacation shame, they're significantly more likely than older generations to say they also shame their co-workers, 42% versus 24%. What's more, millennials who have ever shamed their co-workers were significantly more likely than older generations to say they're at least somewhat serious, 42% versus 22%.

The research also showed vacation shaming is affecting all generations.

Nearly half, 47%, of all workers surveyed said they felt a sense of shame or guilt for taking time off, and 42% of those think their co-workers are seriously shaming them and not just joking. Nearly half of employees said they felt the need to justify to their employer why they're using vacation days.

Alamo's research, however, did not explain why workplace shaming has become a trend.

2. Not Feeling Support

Nearly six in 10 employees reported a lack of support from their boss and more than half sense a lack of support from their workplace colleagues, according to Project: Time Off's survey.

What's more, nearly half of employees surveyed said they sensed a lack of support from their colleagues.

For employees, the boss is the most powerful influence when it comes to taking time off, even slightly more influential than the employee's family.

According to the survey, 80% of employees said if they felt fully supported and encouraged by their boss, they would be likely to take more time off.

"There is a direct correlation between employees who feel strong support from their bosses and colleagues and employee engagement," Project: Time Off said in its research report. "The more support an employee feels, the more likely they are to report higher levels of happiness."

3. Culture of Silence

Employees may not sense support because they work in a culture of silence.

Nearly two-thirds of employees surveyed said they have heard nothing, get mixed messages or have received discouraging messages about taking time off.

According to Project: Time Off,  the culture of silence has created a vacuum and employees have filled that vacuum with pressure they put on themselves.

Nearly a third of employees surveyed said they put a lot or some pressure on themselves to check in with work when they are on vacation, almost twice as high as employees who report feeling pressure from their boss.

One in three managers never talks about the importance of taking time off with their direct reports and another 11% only discuss it once a year, according to the survey.

Managers experience some of the most pressure, which is reflected in their connectivity to the workplace while on vacation.

Forty-five percent of managers said they put pressure on themselves to check in with work while taking time off. In addition, 25% of managers said they feel that their boss expects them to check in while taking time off, according to the survey.

So What's the Solution?

The most important step employees can take is to plan their vacation ahead of when they expect to take off. However, less than half of households set aside time to plan the use of their vacation time every year, Project: Time Off said.

Fifty-one percent of those who plan took all of their vacation time, while only 39% of non-planners did. Sixty-nine percent of planners also were more likely to take a full week of vacation time or more compared with only 46% of non-planners.

Those who took more vacation time also reported less stress at home and more happiness with their significant other and in their professional lives than those who took less time off, according to the survey.

While planning vacation time is important, Project: Time Off also noted that it is important for managers to understand the power of their influence over employees and that companies need to set the tone for making vacation an accepted and encouraged practice.