I think Japanese athletes in general tend to do poorly at the Olympics. For such a wealthy country, they should rank up there with Germany and the USA. But looking at the medal count, that's never been the case.
Part of it is cultural. There seems to be an undercurrent of "You made it to the Olympics, that's good enough", and as a result I think many Japanese Olympic athletes don't really try to get gold. I mean, for Olympic gold you gotta give 110%, and maybe some athletes are only giving 98%. Also, Japan is such a Japanese-centric culture, and glory in the international arena is short-lived and doesn't mean that much to them (IMHO). Tis strange for a country so fanatical about sports.