User blog comment:NeoNirvana/BLEACH 420: Finale/@comment-2041622-20100916082353/@comment-1248963-20100916084035

It is Kubo who writes the story, so it is up to him to explain how Isshin lost his powers. Not us, not you. But this latest development makes it possible that the reason it happend was because he used the Final Getsuga Tensho, that's what people are talking about. After all, Isshin knows about the technique and how to obtain it. Yes, we don't know anything for certain yet, we'll have to wait and see if Kubo will someday draw Isshin's past as he promised he would to finally understand what really happened to make Isshin lose his powers.

As for Ichigo, he himself stated that he hasn't YET lost his powers, but that he WILL lose all of his Shinigami powers after using the Final Getsuga Tensho. Aizen then thought that Ichigo had evolved to a level further than his own, because he is unable to feel his reiatsu, but that has nothing to do with the possibility of Ichigo losing his Shinigami powers in the near future due to using the FGT.