Well I've finished books 5 & 6 of the Dark Tower series, and it's finally dawned on me what's been bugging me.
The first three books are about Roland, Jake, Eddie and Susannah. Books four five and six are at least partly about other people
Book four is completely superfluous to the series, save for the Blaine resolution, Book five (which I thought was good) is at least partly a sequel to Salem's Lot and Book Six is half about Mia.
I guess after seeing them work so well together in the first couple of books, having the next books focus on other people threw me off.
So far I have avoided spoilers and resisted spoiling myself. I hope the last book is good.