The Fitbit, Jawbone Up and Nike Fuelband have helped many Americans track their steps taken and calories consumed each day to take the guess work out of losing weight.

But some wearers are experiencing fitness band frustration. They find that their Fitbit actually moves the scale in the wrong direction — making them pack on the pounds, as opposed to maintaining or shedding unwanted weight.

Fitness social networks and calorie-counting websites have threads asking fellow users if they’re also “gaining weight on Fitbit” and what lifestyle changes or electronic tweaks they can make so their wristbands work for them. One mother posted her excitement when she got a Fitbit for Mother’s Day, only to find she immediately gained three pounds when she started using her new fitness tracker. Confused, she writes, “I’m more active now than ever before.”

Korie Mulholland, 24, a private SAT tutor in Chicago can relate. She lost 40 pounds through a healthy combination of calorie restriction and moderate exercise. But then, as dieters frequently do, Mulholland found that her weight loss plateaued. So she decided to buy a Fitbit to make it over the hump and reach her ultimate goal— especially because she planned on spending much of her summer working on a makeshift desk-treadmill with her iPad.

“Because it tracks steps and calories, I thought a Fitbit would be perfect for me as it got harder and harder to lose weight,” she explained. “And since I was walking 10 to 15 miles a day at my stand-up desk, it told me I could eat 2200 to 2400 calories a day.”

Related: Tracking your fitness? Why 10,000 steps a day may not be enough

But Mulholland said her weight started to go up instead of down — 2 or 3 pounds here and there, as she wore the wristband and followed its calorie guidelines. “I used it for six months, until I gave up,” she recalls. “It was clearly telling me to eat too much for my specific metabolism and no matter what I did, it just wasn’t working right.”

Now, she’s back on target, without a fitness tracking device, losing weight gradually, eating the right number of calories for her specific metabolism, she says, as it varies day-to-day, even if the number of steps she takes may be consistent.

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Watch: Can fitness bands actually make you gain weight?

A Fitbit spokeswoman said the company makes “the most consistently accurate activity trackers on the market," even outperforming heart rate straps and treadmills that calculate calorie burn. “While there may be a small difference of a few calories or steps between tests, ultimately the success of our products comes from empowering users to accurately see their overall health and fitness trends over time,” she said.

Jessica Reed, 38, a poet in Danville, Indiana, had a similar experience to Mulholland when she first got her wristband. On her blog, she called her mysterious weight gain “The Case of the Fitbit Defying Metabolism.”

More than just counting calories

In the first few months she had the Fitbit, Reed was dieting and gained a few pounds, she explained, although she had been consuming fewer calories than she burned according to the device. “I speculate that my weight fluctuations correlate with my greater sense of well-being more closely than exercise habits,” she emailed NBC News.

Experts are unsurprised that some fitness band wearers feel frustrated after they spend $100 on a fitness device and see the scale move in the wrong direction. Sustained weight loss, they say, often involves a lot more than just counting calories. Your overall “well-being,” as Reed puts it, can, in fact, stump your fitness tracker.

“I see people using wristbands, tracking calories, and sometimes the weight just doesn’t come off and they even gain a little with a Fitbit or Fuelband,” said Madison, Wisconsin nutritionist and registered dietitian Margaret Wertheim, author of “Breaking the Sugar Habit: Practical Ways to Cut the Sugar, Lose the Weight, and Regain Your Health.”

Weight loss is more an art than a science. While we might like to think it’s a simple calculation of calories in and calories burned, most of us have numerous, fluctuating variables in our personal weight-loss equation.

“So many people get fixated on the number of calories they are getting every day,” Wertheim explained, “and don’t think about all the other factors that will create a lot of individuality that a wristband doesn’t track, like the kind of calorie you are consuming.” Wertheim says she starts with the composition of a patient’s diet and the first culprit is always sugar and refined carbohydrates, which have a higher glycemic index, causing the body to produce insulin and store fat. “If a person is drinking sweetened beverages or some of the coffee drinks like chai tea lattes, those calories aren’t going to allow them to lose the weight they want,” Wertheim warns.

Hormones, sleep and the time you eat can play key roles in weight loss, too, according to Dr. Holly F. Lofton, director of the NYU Langone medical weight management program.

Richard Drew / AP

“I’m addicted to my Fitbit to keep me abreast of how many steps I’m taking,” confessed Lofton. “But I don’t use it for calorie counting. First, the tracker may be based on flawed or imperfect underlying equation as there are various ones for energy expenditures.”

To get a more accurate reading of a person’s actual energy expenditure, Lofton uses a process called indirect calorimetry. “It’s a simple breathing test done in my office with a portable device that only take 10 minutes or less,” she said. “Because it’s personalized, it’s more accurate than the equations used by fitness trackers.”

Also, she cautions, calorie counting on a wristband doesn’t give you a full picture of your lifestyle— even when you’re tracking your number of steps or hours slept. Are you spreading calories throughout the day or eating a lot before bedtime? Are you much more active some days of the week but not others? And there may be non-food factors at-play, like whether you’ve recently had a baby, or are experiencing high stress levels or a decreased basal metabolic rate during a weight-loss plateau, as Mulholland described.

Fitbit allows people to customize their calorie burn goals, in addition to providing an automated one. "This is to account for people who hit plateaus, or have slower metabolisms than we might estimate," the spokeswoman said.

“There are huge differences in individual circumstances,” said Dana James, a certified nutritionist and founder of Food Coach NYC. Her concern: weight-loss apps and fitness bands don’t take into account bio-individual differences like intestinal microflora and food sensitivities, such as gluten allergies. “If your gut bacteria is full of bad bacteria, you can eat 30 percent less calories, exercise 30 percent harder and gain 30 percent more than someone with the right microbial make-up,” she explained, “and there’s no app that takes that into account yet.”

Article was originally published in June 2014.

So if you have a fitness band, James advises using it wisely. Record what you eat, and how frequently you exercise, but ignore the calories consumed and burned. Over the long haul, for many dieters, weight loss is still too complex for even the niftiest tracking device.

This story was originally published in July, 2014

Jacoba Urist is a journalist in NYC, who covers health, education and gender issues. Follow her on twitter: @JacobaUrist