This is deeply unpopular but for me it was The Ocean at the End of the Lane by Neil Gaiman. I just didn’t get what was happening half the time; whereas with some books like The Master and Margarita I’m confused but pleasantly along for the ride anyway, for this one I was just kind of like “that’s it?” by the end. But I think Gaiman just isn’t for me - have also read American Gods and Coraline and he’s an objectively great writer, just not my style.