According to this article, http://www.sciencemag.org/news/2016/02/gravitational-waves-einsteins-ripples-spacetime-spotted-first-time, the search for gravitational waves was beset by continual failures. Even the LIGO took a very long time to put up a positive result. They even had to upgrade the LIGO, which was presumably already quite sensitive.
Why did it take so long? Are gravitational waves smaller than expected? Or is there some other difficulty with detecting them that was unanticipated? If so, how did physicists fail to predict these things?
I'm not asking why gravitational waves are hard to detect, but rather why they were harder to detect than expected.