Natsume's Book of Friends

From the day where a picture of a row of cherry trees in a desolate winter was obtained, every time Natsume wakes up, his room is covered in petals. Trying to solve the root of this activity, Natsume and Nyanko Sensei try to return the picture to Miya, a shadow figure who accompanies the picture. But why can't they return it to Miya? (Source: Wikipedia)