Mucus collects in the nose, is difficult for baby to clear and that leads to blocked nose. Generally babies aged less than 6 months have a blocked nose (commonly called 'snuffles'). No treatment is required if the baby is otherwise well and feeding well. Try placing a bowl of warm water in the room where the baby sleeps. This raises the humidity which may help to loosen thick mucus. Try giving smaller but more frequent feeds. If the above measures do not help Saline drops or sprays may be useful. Saline drops act as a decongestant which relieves congestion of nose.