Kelly Azevedo used to work upward of 70 hours a week.

It wasn’t really for the extra money. It’s just that she doesn’t have children. When colleagues were on kid duty, she had to pick up the slack.

“Parents are a special class, and they get special treatment,” says Azevedo, 27, who left her job at an Internet marketing firm to start She’s Got Systems, a website for entrepreneurs based in Sacramento, Calif. While she was covering for her former colleagues, Azevedo says, she sometimes sacrificed her own obligation to take care of her ailing grandparents.

Azevedo is giving voice to what many people feel in their bones: The pursuit of “work-life balance,” which sounds so wholesome and reasonable, can be a zero-sum game in the office.

In theory, flextime seems like an everyone-wins proposition. But one person’s work-life balance can be another’s work-life overload. Someone, after all, has to make that meeting or hit that deadline.

As a result, many Americans who work for companies that embrace flexible hours are confronting a sort of office class warfare. Some employees have come to expect that the demands of their children, in particular, will be accommodated – and not all of their colleagues are happy about it.

These tensions are hardly new. But at a time when many Americans are struggling to find or keep jobs – and when many of us are being asked to do more with less – the issue has come to the fore.

Child care has long been the third rail in this conversation, and it is receiving renewed attention in no small part because of a recent article in The Atlantic by Anne-Marie Slaughter. She discussed the moment she realized that she was unable to hold down a high-level State Department job and attend to her two adolescent boys. While advocating workplace flexibility for everyone, Slaughter stressed the special problems that women face balancing their careers with children.

Slaughter, 53, says both mothers and fathers should be “open and indeed proud” to leave early for the sake of their children and deplores the notion that spending more hours in the office automatically translates into getting more work done. But sometimes there is no substitute for office face time.

It’s not just the moms who are juggling.

“My kids have swim classes that start at 5 p.m.,” says Aziz Gilani, 32, a director at DFJ Mercury, a Houston-based venture capital firm. “The net result is that I’m sure there are times when my partners are expecting me to be in the office and my office is empty because I’m doing one of these parental commitments. I’m sure it creates a burden for them.”

Gilani says it helps that two of his four partners also have young children, but that also means he’s often on the other side. “Sometimes I need an answer immediately, because what we work on is often time-sensitive, but my partner is at soccer practice with his daughter and that has created a decent amount of inconvenience.”

“It’s rough,” he says, but office technology enables them to work around the problem.

Gilani says there has never been a major spat at his firm over the issue. But Deborah Epstein Henry, founder of Flex-Time Lawyers, has found that colleague resentment is very common. “It’s the reason that a lot of work-life balance programs fail,” says Henry, whose firm, based in Ardmore, Pa., advises law firms and other organizations on flexibility policies. “In an ideal world, no one else is saddled with more work if their colleague works a reduced schedule.”

‘De-parenting’

So what should an employee tell the boss when life bumps up against the job? “I think the default is to focus on, ‘Where am I going?’ ”says Cali Williams Yost, who has advised the United Nations, Microsoft and Johnson & Johnson, among others, on flexible work strategies.

“Instead, employees should focus on, ‘How am I going to get my job done?’ ”

Problems with work overload often stem from poor implementation, Henry says.

Companies should put one person in charge of overseeing all of the employees who are working these compressed schedules, tracking their hours, and looking at the assignments and how they are staffed, she says.

“An effective work-life program is one where an employee gains flexibility while continuing to be responsive and accessible to colleagues and clients,” she says.

Often, the culprit in these situations is lack of communication, says Williams Yost, the author of the forthcoming book “Tweak It: Make What Matters to You Happen Every Day.”

Employees often do a better job of communicating and coordinating with their supervisor than with their direct reports and co-workers, according to Williams Yost, whose firm commissioned the survey, which is done every two years.

“Without communication, our flexibility program would not work,” says Blair S. Murphy, 40, a tax partner at Ernst & Young, the global accounting firm, in Boston.

Murphy works closely with a female colleague who leaves every day at 5 p.m. to relieve her child care provider.

He, in turn, leaves early two days a week in the spring to coach his son’s Little League team.

“Our group is constantly communicating,” he says. “Everyone basically knows where everyone else is at all times.”

So what’s the solution?

Williams Yost says we need to start by “de-parenting” and “de-gendering” the conversation.

“You may cover for me during my son’s soccer game, but I may cover for you while you take your mother to the doctor,” she says. “That’s how we can stop overburdening people,” she adds.

“No good flexibility policy ever put in place was for parents only.”

But even well-designed policies cannot always accommodate the reality that many industries are deadline-driven or not particularly conducive to balance.

Fixing the environment

At Ernst & Young, the policy is that everyone – no matter their age or life circumstances – has equal claims on flexibility – there is no work-life balance trump card, says Karyn L. Twaronite, Americas inclusiveness officer at Ernst & Young in New York. While implementation varies slightly by practice area and group, employees, whether they play in a weekly basketball league or need to pick up their children from school, can mold their schedules, to a certain degree, to fit their personal lives.

“When we first gave this a shot back in the 1990s, it did have the tendency to build resentment on some teams, but we moved away from fixing it as a ‘woman problem’ to fixing the environment,” Twaronite says. “We also didn’t want working parents to feel embarrassed about taking time off.”

Who gets priority, though, if one employee’s son has his back-to-school night on the same night as another’s poker game?

“People switch on and off, and there is a lot of transparency,” Twaronite says. She took advantage of the company’s flexibility program to leave early two nights a week to attend business school at Fordham University from 1992 to 1995 and to take off a few Fridays a month when her son was younger.

Murphy says there are times when he has to pick up the slack between 5 and 7 p.m. because his colleague leaves at 5. But it’s a two-way street; his colleague covers for Murphy, too. “I don’t feel like I’m working more because of someone’s else’s flexibility,” he says, “I just feel like I’m working differently.”