Originally Posted by kael03
Yama explained that, too. He figured they couldn't steal someone's bankai if they didn't know its full strength. Since Ichigo's bankai still has some evolving to go, they don't know its full strength.
Therein lies the problem with this manga, Renji's has developed supposedly, one would think Rukia, Kira (should've before he perished), Rangiku, and prolly Hisagi would have Bankai's and they couldn't be known. The whole set up of the stealing of the Bankai's comes off just wack, without specific rhyme or reason. Since the beginning of the manga, Ichigo is the only one that has 'achieved' Bankai from my knowledge. Ikkaku and his side homey has had theirs hidden since forever.