I can't gain my weight I'm thin

Causes of non-gaining weight

Lifestyle
Many people who can't gain weight don't realize they are not getting enough calories to gain weight. Skipping meals due to a busy schedule or simply not consuming enough of the right foods, such as fats or carbohydrates, can be factors. Exercising but not consuming enough calories to make up for the expenditure is another common reason you might not gain weight. Monitoring your daily caloric expenditure and intake can help determine if this is the case
Hyperthyroidism
Diabetes
Gastrointestinal Issues
Problems with the intestines can mean your body is not properly absorbing the food you are eating. Even if you are consuming a lot, you could suffer from malnutrition and not be able to gain weight. These types of diseases are known as "malabsorption syndromes." Some common forms of this disease type are irritable bowel syndrome, Crohn's disease, ulcers and celiac disease, according to the Mayo Clinic. Symptoms can include blood in the stool, abnormal bowel movements, fatigue, pain in the abdomen, gas and possible weight loss. If you are experiencing these symptoms along with no weight gain, consult a doctor for proper diagnosis
Other, less-common, issues that could be at fault. Some prescription medications can increase your metabolism, which will burn more calories. Cancer, infection and neurological issues can all affect your weight, usually in the form of weight loss
Advice
It's best to consult a doctor to get a proper diagnosis. Even if it ends up that you are one of those who simply has a hard time gaining weight, knowing what is at the root of the problem can offer relief and give you an indication of what your next step should be