Chiming in with reminders, data, and tips, our sleek gadgets and handy apps want to program us into being better versions of ourselves: more responsible, productive, healthy. But, sadly, some technology is no match for the chaotic code of an emotional human—particularly one struggling on a diet.

According to a two-year study, wearable fitness trackers designed to coax users into busting moves and burning calories throughout their daily lives didn’t help anyone lose weight. In fact, overweight dieters using the arm-mounted gizmos actually gained more weight on average than those using old-fashioned, tech-less dieting schemes. The study, published Tuesday in JAMA, contradicts earlier studies that found the trackers can boost weight loss. But those earlier trials tended to be smaller and shorter.

The new data, the authors say, suggests that tossing technology at big problems, like fitness, diet, willpower, and motivation, isn’t straightforward and requires more nuanced, long-term studies. “I think we have to be a little bit cautious about simply thinking that what we can do is just add technology to these already effective interventions and expect better results,” lead study researcher John Jakicic, of the University of Pittsburgh, said in an interview with JAMA.

For the study, Dr. Jakicic and his colleagues started with one of those effective behavioral interventions. They enrolled 471 young adults (aged 18-35) who were overweight (with an average weight of around 210 pounds) and wanted to slim down. For the first six months, the participants had to stick to a low-calorie diet, a prescribed fitness plan, log their progress in diet diaries, and attend weekly group counseling sessions.

After six months, everyone had lost weight—about 17-19 pounds on average.

Next, the participants were divided into two groups. One group got the fitness tracker for 18 months, while the other just had to log their activity into a study website (considered a standard dieting method). By the 24-month mark, many participants in both groups had regained some of the weight they lost in the first six months. Those on the standard plan were, on average, 13 pounds lighter than when they started the whole thing (before the six-month intervention). But those using the fitness trackers were, on average, only about eight pounds lighter.

While the results surprised the researchers, the data didn’t provide any clear clues as to why the fitness trackers seemed to sabotage dieters’ weight loss efforts. Perhaps the devices worked to get people moving, but then led them to be hungrier and overeat. Or it’s possible that people might have felt discouraged if they kept track of their fitness each day, felt they weren’t going to meet their daily goal, and then gave up early.

Jakicic says future studies will be necessary to tease such potential factors out, plus test the effectiveness of different wearable fitness tracking devices. “Probably more importantly,” he said, “is for us to try to understand for whom and when these devices are actually very effective.” For some people, fitness trackers might work, he said. For others, they might backfire.

JAMA, 2016. DOI: 10.1001/jama.2016.12858 (About DOIs).