User blog comment:Xilinoc/Ch. 569 (You've been hit by / You've been struck by / Senbonzakura~)/@comment-7079377-20140221113541/@comment-10554972-20140222034415

Hisagi having Bankai makes a lot more sense than Rukia having it, because he could have been training for Bankai since the start of the series or so. I think that by the time he did the training with Kensei, he already had done enough training to properly use Bankai, but he just didn't have the confidence to use it, or something to that effect. That's why he didn't use it in the first invasion, he didn't think he was capable yet. The training with Kensei was to get him to use it or die. That's why Kensei simply told him to use Bankai.

Rukia however has only recently been a lieutenant so I find it doubtful that she would have been training for Bankai for very long, if at all before the Royal Guard training. It's not like you just immediately start training for Bankai as soon as you become lieutenant. And yes, you can train for Bankai before you are lieutenant, but Rukia wasn't that strong at the start of the series and she definitely wasn't confident enough to start her training back then.