Not every child grows at the same rate, of course, and growth charts aren’t perfect. But they exist for a reason: They provide strong guidance on the expected physical development of children as they get older.

If your child consistently falls on the low end of the chart—or they’re losing weight—you’re probably wondering what it means. Two popular growth charts can help you track your child’s physical development. The CDC recommends its growth chart for children age 2 and older, and refer to the World Health Organization's growth standards for children under 2.

Slow or stagnant weight gain or growth is known as failure to thrive. It’s not a medical condition, but rather a state of undernourishment. Solving the problem sounds simple: Get the kid to eat more. But odds are you’ve tried that, so let’s dig into the real reasons why kids don’t gain weight and what you can do to get growth back on track.

Why your child may not be gaining weight

Taking in and absorbing more calories should help your child gain weight, but the fix is seldom that simple. In more than 90 percent of cases, lack of calories isn’t the sole issue, according to Children’s Hospital Wisconsin. Instead, a mix of environmental factors and potential medical conditions may be the culprit.

John’s Hopkins Medicine segments potential explanations for failure to thrive into medical causes and contributing factors. As a parent, those factors are things you’d already know about: Economic problems that prevent proper nutrition, an unloving home or hostile relationship between parent and child, and infections or toxins the child may have been exposed to.

Types of food and the timing of meals can influence growth, too. The NHS recommends that parents not feed their children whole-grain foods with high fiber content before the age of 2. Adults may choose those foods for themselves to stave off cravings, but high-fiber meals can satiate your child before they’ve taken in enough nutrients.

Filling up on liquids can have a similar effect, so monitor your child’s juice intake to make sure they’re not skipping out on crucial foods. Limit juice to less than 4 ounces a day, suggests Vanessa Millovich, DCN, RDN, a registered dietitian with Kate Farms.

Meanwhile, the possible medical causes are manifold. Issues with the endocrine system, including thyroid deficiency or growth hormone deficiency, may be stunting growth. Or an overactive thyroid can cause excessive calorie burn. Oral sensitivities, caused by a cleft palate or cerebral palsy, may prevent a child from swallowing properly.

Some underlying causes prevent food from staying down or being absorbed properly. Severe acid reflux can lead to excessive vomiting. Bowel disorders like celiac or Crohn’s disease may also impact nutrient absorption.

Last, existing medical conditions not directly related to digestion may have a secondary effect. Chromosome abnormalities like Down syndrome and Turner syndrome can slow growth, as can lung and heart conditions that disrupt the movement of nutrients and oxygen throughout the body.

When to worry about your child’s weight

A single drop in weight isn’t as concerning as a trend of low weight over time, according to the Cleveland Clinic, so don’t worry too much if a period of consistent growth is followed by a brief lull.

However, failure to thrive may exist when a child’s weight slides below the second percentile of the WHO growth standards or the third percentile of the CDC growth chart, according to Children’s Hospital Wisconsin. And John’s Hopkins Medicine notes that a child who is 20 percent below their ideal weight for height may also be experiencing slow growth.

Observant parents may also notice that a child is slow to develop skills such as walking, standing, sitting, or rolling over. Social and mental skills may also be impacted by failure to thrive. And lack of nutrition can affect a child’s disposition, making them fatigued and irritable. In other words, see a doctor if you notice any of the those indicators.

What to do if your child is failing to thrive

Make an appointment with your child’s primary care physician. Because failure to thrive is a complex issue that seldom has to do with parental negligence, parents shouldn’t be concerned about seeing a doctor regarding their child’s low weight. Conversely, a doctor’s evaluation may be crucial in identifying the underlying causes of slow growth.

A physician may evaluate your child’s growth based on a handful of factors, according to the Children’s Hospital of Pennsylvania. Age, health, and medical history will be taken into account, as well as the severity of symptoms and the child’s tolerance to various medications and therapy modalities.

When dietary fixes are appropriate, doctors may recommend you find ways to add calories to your child’s diet. The needed calorie increase may be subtle: as little as a 5 to 10 percent increase of daily calories, says the Cleveland Clinic. That said, if a child requires catch-up growth, your doctor may recommend 20 to 50 percent more calories per day from baseline, says Millovich.

Don’t immediately spring for sugary foods, which can have adverse health effects and ruin your child’s teeth. Instead, high-fat, energy-dense foods can provide the concentrated energy needed for growth. Such foods may include whole milk, yogurt, butter, oil, and special formulas designed to provide the necessary nutrients in an effective dose.

Treatment may last a few months to a year, according to Children’s Hospital Wisconsin, so be patient and schedule regular check-ins with your doctor to keep track of your child’s progress.

Failure to thrive can have lasting effects on a child’s development if it isn’t addressed immediately, so don’t hesitate to see a doctor if you’re concerned about your child’s growth. A pediatric dietitian can help you find ways to add the needed calories to your child’s diet—and suggest behavior changes to ensure your child receives the nutrition they need to thrive.