None of the "Descent" Borg were meant to be assimilated individuals, they were all supposed to be Borg that were Borg from infancy. They were meant to be the adult form of those baby Borg we saw in "Q Who?", ones who were never individuals in the first place.
That's why none of them regained their former individuality and why they were so susceptible to Hugh's individuality in the first place.
Lore promised he would make them all 100% robotic beings in exchange for their loyalty, of course he didn't know how to do it. That was why the Borg he experimented on were just deformed cripples.
Source for this?
It's the only thing that makes sense given what we know about the Borg. Otherwise they all would've regained their individuality once they were cut off from the Collective (and they WERE cut off) just like Picard and the Borg from "Unity".