Board Thread:General Discussion/@comment-4957813-20131130185522/@comment-1268838-20140930204729

Renji has not even possessed his Bankai for three years; Byakuya stated very early on that fully mastering it normally takes ten. It may be easy to forget, but as powerful as he is/was, Renji is still a neophyte to this captain-level business. It is perfectly reasonable for him to have only possessed "half" of his Bankai. Yamamoto himself stated that young Bankai could evolve over time.