f you mean their fight in Soul Society arc, Ichigo was already exhausted after fighting Byakuya. That would cause his attack to be less effective. It is also later revealed that Aizen has at least twice the reiatsu of an average captain. With that said, it is possible for Aizen to stop Ichigo's assault on him. According to Aizen : "Battle between Shinigami is a battle between reiatsu. And my reiatsu will crush yours" [to Soifon] . Aizen can block with one finger Ichigo's Bankai beacause he has a vast reiatsu. That same goes on Komamura's shikai and Bankai and Soifon's Shikai.