I mentioned this in the early pages of the thread, but I think this leads back to Isshin's statement from past chapters about how Aizen's spirit power was so high he, Urahara and Yoruichi couldn't feel it.
Ichigo was the only one who could feel it and understood how vastly they were outmatched (hence why he was shitting his pants).
Now it is HIS (Ichigo's) spirit energy that so far excels the others that even Ai-chan can't feel it.
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