Not necessarily thrillers, sometimes a book just doesn't grab me. But actually the first example that comes to mind is a thriller. It was a new take on the Philadelphia Experiment folk tale, with people in the present continuing the research and dealing with the side effects. But about halfway through, I just wasn't feeling it, and DNF'd.

Now that I think about it, the thriller depends pretty heavily on good pacing and gripping prose, to keep the reader's interest and excitement throughout the story. So I guess maybe thrillers are less forgiving of author missteps. Other genres can better afford to lose some momentum, or just shift gears here and there. But a thriller needs to keep on banging away at full speed, or it just falls apart.