I remember that in season 3, it took the show ages to reveal who is the "boy" (Ramsay). Now they are doing the same thing with the dagger. It was so brilliant of Bran to figure that whoever hired the catspaw assassin and armed him with the dagger was wealthy, but couldn't he use his powers to see who it was? After all it happened in Winterfell, where there are weirwood trees.

For those who have not read the books: in book 3 Tyrion and Jaime figured who hired the assassin.