Board Thread:Questions and Answers/@comment-6207546-20130504124857/@comment-11051098-20140703181102

Isshin was a captain around 20 years ago at the time he "deserted", I think it was said. Rukia only became a lieutenant during the last 17 months, and wasn't even a seated officer prior to that.

Being unseated probably means she mostly stayed with her own squad, and since it takes time to get to another squad's barracks, she might just never have seen him when he was a captain, as Stefan says.