Probably when they decided to introduce a billion side Ninja who's presence is ultimately trivial and are mostly around to accompany Naruto into his various romps into Filler Land. It's a problem that Bleach has as well, they introduce far too many Shinigami and try to give all of them their own screen time and it just doesn't work out. Oh yeah, the part where they turned Naruto into a God and ressurected every villain in the series didn't help much. And Kishimoto can't create strong female characters.