I'm sure it happens to everybody, and it's really a shame when it does. You see a book, you pull it off the shelf, and you get really excited about it. The cover is interesting, the premise sounds promising, but a few chapters in, you realize you're forcing your way through the pages.
It sucks, but it happens, and it may be for a number of reasons... you're busy, you find a more interesting book, the library won't let you renew it anymore, or the worst of all, it just doesn't capture your attention.
It makes me wonder what it is exactly that makes a book lose someone's attention. Obviously, the book was good enough to make it to print. And it was picked for a reason. Most of the time, it's surely just the personal preference of the reader. And sometimes, like with me, the reader jumps to unrealistic conclusions, and expects the book to be something completely different -- even something that contradicts the summary on the back of the book.
Take for example one book I've only gotten halfway through. I went into it knowing that it would be about alchemy and that it would take place in the past. But I automatically assumed it would a version of Frankenstein. I thought a person would be brought to life, but it was only a scientific endeavor with a rose. There went my interest.
Then there was another. This one I didn't have unrealistic ideas sprouting out about it, but it lost me a little over a third of the way through. There was something interesting about it, but the writing, and the droning, left me itching to get to the good part, and I couldn't force my way though the pointless text to get through it.
I wish there was a scientific formula for figuring out why books fail to catch some people's interest, but it's true that it's all based on opinion. It's a curse we all face when we write. The publisher might like it, but it doesn't necessarily mean that the public will.