User blog comment:Xilinoc/Ch. 561 - THE VILLAN/@comment-6657474-20131212010909

On the subject of why the Visored Captains haven't hollowfied; it's pretty obvious that they still have the ability to hollowfy, as Mashiro demonstrated earlier, however, it's possible that the reason they haven't in the recent battles may be because they're not allowed to use their hollow powers WITHIN the Serietei. If you remember, when Ichigo and co. first entered the Soul Society, the Shimigami needed special orders to be able to carry their Zanpakto within the Sereitei. Maybe part of being allowed to return to their former positions was that they had to agree never to use their hollow powers within the bounds of the Serietei, since it translates as "Court of Pure Souls", and hollows are considered by the Soul Society as anything but.