Babies who do not gain weight on breastmilk can be caused by insufficient breastmilk secretion, lactose intolerance, protein allergy and other factors. 1. Insufficient breast milk secretion: When breast milk secretion is insufficient to provide enough calories and nutrients to support the child’s weight gain, the child may not gain weight. 2. Lactose intolerance: Breast milk contains more lactose, if the baby has a congenital lactase deficiency and is lactose intolerant, the baby cannot effectively absorb the nutrients in the milk powder, resulting in weight loss, usually accompanied by diarrhea, abdominal pain, nausea, vomiting, and other symptoms. 3. Protein allergy: When the mother eats soy, seafood, milk, eggs and other foods, may lead to the baby to protein allergy, thus the situation of weight gain, usually accompanied by mucus blood stools, abdominal pain, abdominal distension and other digestive symptoms. There may be other reasons why your baby is not gaining weight on breast milk, so it is advisable to consult a doctor to find out the cause of the problem and treat it accordingly.