Board Thread:General Discussion/@comment-27458826-20180127131846/@comment-26912075-20180214123539

Petentro wrote: Oh and if you will recall when Renji fought byakuya and was severely wounded his bankai reverted to shikai. Byakuya said that a bankai reverting without permission was a sign he was close to death. Ichigo had a hole blasted in his chest by Ulquiorra and his never reverted

Considering all the plot inconsistencies in the series, this isn't surprising.