1. Cleft lip or cleft palate: Cleft lip or cleft palate means that the baby's mouth or mouth is not completely closed at birth. This may cause the baby's mouth not to be completely closed.
2. Tooth problems: If the baby is teething or has tooth problems, such as buckteeth and teeth dislocation, it may also lead to the mouth not being completely closed.
3. Muscle problems: The baby's mouth muscle dysplasia or muscle disease may also lead to the mouth not being completely closed.
4. Nervous system problems: the baby's nervous system is abnormally developed or has nervous system diseases, which may also lead to the mouth not being completely closed.
5. Other reasons: The baby may not be able to completely shut his mouth for other reasons, such as fatigue and respiratory infection.
If it is found that the baby's mouth cannot be completely closed, it is recommended to seek medical advice in time, and a professional doctor will check and diagnose it, determine the specific reasons and formulate corresponding treatment plans.