Board Thread:General Discussion/@comment-5695542-20131107215511/@comment-8986886-20131124123210

Savalric wrote: Kenpachi is awefully powerful, enough so to scare yamamoto into never training him (fully). I don't think someone who couldn't handle Yamamoto's bankai could handle Kenpachi.

Unohana was playing with Kenpachi down in the Muken, mortally wounding him then healing him over and over, forcing Kenpachi to use more of his hidden powers. That fight took place AFTER Kenpachi's fight with Royd, so he clearly wasn't using his full potential at the time.