Did you have the vet thoroughly check his teeth?  Does he have a chipped
tooth, you know, the 4 long fang looking teeth in the front?  Does he have
a loose tooth or darkened teeth of any sort in his mouth?  Sometimes you
have to look very closely and carefully to see the chips and breaks in
them.  I have had ferrets in the past with teeth problems and they can be
fed soft food but cannot "crunch" on their own till the tooth problem is
fixed.
 
I learned the hard way after trying everything but the teeth, even my vet
missed it.  I just thought about it one day and realized maybe a sore tooth
or a mouth sore would make the ferret not want to eat normally.  Since I
saw no mention of teeth in your post that is what I thought of right away!
 
Good Luck,
Joey
[Posted in FML issue 3017]