User blog comment:Xilinoc/Ch. 564 (Mask is too fabulous to live)/@comment-2120712-20140118014248

Now I see what Byakuya means about needing 10 years to master bankai, because even after subjugating your zan spirit to gain a bankai, it may still be holding out. I guess after mastering unrealized bankai or something the zan spirit entrusts you with its true final form.