One suggestion that might work to help your ferrets learn their names: say
their names in a different tone of voice.  W/all four of my ferrets I've
always said their names in a different tone than my conversational tone,
usually more high-pitched.  But also I say each name differently as well.
It's worked w/all of them, if for no other reason than the sound that comes
out of my mouth is different than the one they are used to.  But if I call
"Zooey", Linus doesn't turn his head.  But he will if I call his name.
 
Maybe that's also b/c I talk to them incessantly...
 
Minta
(& Linus, Martin & Zooey)
[Posted in FML issue 2222]