Board Thread:Improvements and Issues/@comment-5355747-20130313062554/@comment-8986886-20130721101636

Maybe Shinigami in general are bad at recognizing change in appearances, since everyone ages very slowly in Soul Society. Isshin aging 20 years in the human world and growing a beard, would probably be equivalent to him living 200-400 years in Soul Society (it seems to vary between individuals)

The fact that Isshin lost his Shinigami powers and didn't give away any Reiatsu also did it's part to keep Rukia in the dark.

(Kyūkyoku KON> When I missed my buss last week I always suspected it was Aizen's doing; he must have planned it all along. Curse that man!)