I won't give anything away but I think the pace of the book is a bit off. In the other books you get a sense of the seasons throughout the Hogwarts year. In this one something else happens but they seem to not be doing a lot for stretches at a time. It's like the first 400 pages are Rowling not wanting to finish it and then in the last 200 pages it gathers serious momentum

Not the ending I was expecting either