When authors don't fulfill their contract (with the reader)

Anyone reading these posts may notice some sort of pattern: all too often, it seems, I start a book with great enthusiasm but then half-way through or even later my interest begins to flag and my hopes are dashed. Is this really unusual? I think we always start reading a book with a positive, hopeful attitude - like the first day of a new (school) year! Why else would we pick up a book (other than an assigned text perhaps - and even then we'd open it with hope)? Novels begin with a sort of unspoken, unwritten, but mutually acknowledged contract: the novelist introduces a character or some character, a setting, and hint of a theme, a voice or style - and we expect some reasonable degree of consistency throughout the book - even if it's an experimental or unconventional book, we expect that unconventionality to be consistent. We also expect the characters to develop, become more round (as Forster put it), whole, or real, whatever that may mean, and we expect a plot to develop - the character(s) grows as they take actions and as their world acts upon them - sometimes in surprising ways. The character must face some kind of crisis, dilemma, decision - they can't just drift from event to event or there's no story at all. If the novelist doesn't fill these elements of the "contract," we are disappointed and often we/I abandon the book, and why not? Sometimes, though, it takes quite a while to know whether the book is going anywhere - we put a lot of faith in the novelist, and, depending on how well we know his or her work and on our commitment to the work in hand, we keep going, thinking, yes, all of this will eventually come together, all of this will make sense, this will pay off later. All these thoughts - a long way of saying I am at or near the point of abandoning A.S. Byatt's "The Children's Book," which started so well but 300+ long pages into it is just meandering along going nowhere - and I've just about had it with the tedious description of about 15 characters wandering through the Paris exposition looking at ceramics. Aside from a very specialized audience, who would read this stuff? Did critics really love this book, as the blurbs/reviews imply, or were the awed/cowed by Byatt's magisterial reputation and by the daunting intelligence?