Since Bran became a cripple in Game of Thrones and began seeing the three-eyed raven in his dreams, Maester Luwin insisted on it being just dreams and nothing much.
We can believe that an old knowledgeable man wouldn't want to worry a kid about these things even if he knows those are really, he could probably be thinking that it was just a dream, but it happened not just once and Maester Luwin wanted Bran to believe that these are nothing but just dreams. He even told Bran that he tried his hand at spells like many Maesters do but failed.
Did Maester Luwin really think that Bran's dreams were nothing but just coincidences, or he wanted to keep these things away from him to protect him, as he might have sensed that it could possibly be dangerous for him to discover and use his powers?
As a Maester, he might be more knowledgeable in this field than a general person, assuming that he had no idea about it is quite difficult.