Sometimes the magic leaves a project. We don't get to the ending, or maybe we don't get to turn that early draft into the book it could be.
Of course, maybe it was never meant to be a book in the first place.
My files are full of projects that I took through a few chapters or a few drafts, but then abandoned. "Abandoned" may be a strong word, however. Only when I'm dead will they be truly and permanently abandoned. For now, they're just in suspended animation. Because I frequently do return to older projects when I'm ready--when I've finally figured out how to write them, or when I can stand to be around those characters again, or when I can handle that subject matter.
It's common to hit tough spots with manuscripts. The question is how to tell a tough spot that must be powered through from a tough spot that means the project needs to be set aside.
For me, it's a gut-level decision. I could say that boredom or cluelessness (no idea what happens next) are signs to abandon a work in progress. But I often hit those spots with manuscripts I'm passionate about, and it might just mean I need to backtrack a bit, or delete a bad scene, or do some character sketches to identify the motivations. A more fatal flaw is the lack of a compelling voice--but then, the project might be resurrected with a different main character.
Deep down, it's just a feeling of excitement, of commitment, a sense that this manuscript is worth it. That this story is interesting enough to finish, and revise, and revise again. That my life will be just a little bit less if I don't finish this story.