User blog comment:Xilinoc/Ch. 564 (Mask is too fabulous to live)/@comment-117598-20140115180531/@comment-3250274-20140116172233

Just look to the section on bankai that reinforces the need for additional training to master after initially obtaining it. Trust, or lacking knowledge of the full Zan name may be a factor of this additional training.

Bankai (卍解, Final Release) is the second and final upgraded form of a Zanpakutō. To achieve Bankai, one must be able to materialize and subjugate their Zanpakutō spirit. Materialization means the opposite of getting dragged into the Zanpakutō's inner world: the wielder needs to summon the Zanpakutō's spirit into the physical world. It usually takes 10 years or more to achieve, plus the experience needed to master it. ...... Weakness: The biggest flaw of Bankai lies in the overwhelming power it releases. Its power and form are often far beyond those of a normal weapon, which is why one needs to train at least 10 years with Bankai in order for one to become capable of using it completely and effectively.