If the scale is stuck, don’t automatically blame your weak willpower. There are a host of surprising things that can keep you from slimming down. We’ve highlighted six diet problems that can send you into eating overdrive and the easy ways to spot and overcome each one.

1. Snack packs.

There’s complicated psychology at work in those 100-calorie snack packs at the grocery store. They are based on something consumer scientists call unit bias: The bigger the size of the serving of any given food, the more you will eat of it. Reduce the size of the package, then, and it seems like you would eat less. Right? Not if you’re a dieter.

A study from Arizona State University found that non-dieters consumed fewer calories when they ate M&Ms out of a mini-pack, but dieters consumed almost twice as many calories when they ate M&Ms out of snack packs compared to eating out of traditional packaging.

“Consumers perceive the mini-packs to be diet food,” the authors wrote. “For chronic dieters, this perceptual dilemma causes a tendency to overeat, due to their emotion-laden relationship with food.” In other words, the snack-packs seemed “safe,” so dieters felt free to eat more of them.

Another study, from Tilburg University in the Netherlands, that tested the effects of packaging size on the consumption of potato chips in college students came to the same conclusion. The Dutch study found that participants were more likely to regulate their eating when they opened a large bag of potato chips instead of a snack pack. Both studies were published in the Journal of Consumer Research. 

Slimming solution: Instead of stocking your pantry with 100-calorie snack packs, use a plastic baggie to implement your own portion control. Measuring food yourself will force you to be conscious of how many calories you’re eating and reinforce the decision to stop when you hit your pre-planned limit.

2. Diet soda.

Regular soft drinks are the biggest source of calories in the American diet, and numerous studies have tied them to weight gain. Yet, even the calorie-free kind may lead people to put on pounds. Researchers found that each can of diet soda a person drinks daily increased a person’s risk of becoming obese by a whopping 41 percent. The study was published in the journal Obesity.

“We didn’t prove that it causes weight gain, just that it’s connected to it,” notes study author Sharon Fowler, faculty associate in the division of clinical epidemiology at the University of Texas Health Sciences Center in San Antonio, though she admits she doesn’t know why diet soda might cause weight gain. “One possibility, which we haven’t studied yet, is that there’s a chemical in diet soda that increases appetite,” she says.