User blog comment:Xilinoc/Ch. 670 - The Perfect Crimson/@comment-25897574-20160415040047/@comment-3423279-20160415133942

Isshin explained while Ichigo was in Jinzen that Zanpakutou don't like to teach their wielder techniques or something like that, and you need to fight them to learn.

I would presume this means that until you complete Jinzen entirely you still don't know the limits of your Shikai or Bankai, and Toshiro might of only learned of this capability or only achieved it recently.