User blog comment:Xilinoc/Ch. 670 - The Perfect Crimson/@comment-25897574-20160415040047/@comment-25897574-20160415093151

Yes but Renji was only relatively new to having a Bankai and hadn't trained long enough with it to get that far on his own. He needed to be put through special training to forcibly bring out it's full potential, otherwise his full abilities with it would have only been as far as the giant snake skeleton. Toshiro has had Bankai for much longer so it only took a couple of years of intense training to achieve his full Bankai, but before the time skip, his limits were what they were before the training. We've seen these characters be pushed to their (at the time) limits. Besides, wouldn't Toshiro's Zanpakuto Spirit tell him about it's full capabilities?