Had it not been for Ulquiorra smashing one of the horns on Hollow Ichigo's mask before it could unleash its Cero on poor Uryu, Ichigo's mask would not have been cracked open by the explosion and he probably would not have reverted back to his old self.
When Ichigo returned many chapters later to face off against Yammy, he summoned forth a new Hollow Mask that bore reminiscence to the monstrous Hollow he became during the fight with Ulquiorra, one that he would later note being much heavier than the previous masks.
Now, while this is all just speculation and theory, what if the reason as to why Ichigo cannot summon his mask anymore is because he is no longer a Vizard?
Think about it, Ichigo was nothing more than a mindless Hollow and that mindless Hollow got its mask ripped off thanks to Ulquiorra's actions.
Now, as we all know, when a Hollow's mask is ripped off they usually end up transforming into Arrancar. Can you see what I am getting at? I think the reason for Ichigo's power fluctuations and the reason why he is now unable to summon his mask is because he isn't a Vizard anymore, he's an Arrancar.
What do you all think? Is it possible that Ichigo is turning into an Arrancar and cannot yet use his Hollow powers because his body has not adjusted yet or do you think that there is another reason for his power fluctuations?