That is bullshit.
The most reliable jill I ever had never ate rabbit even once. She simply didn't recognise it as food. But she accounted for a great many and was pretty beneficial as she would never sit on a kill.
And if you actually think there's a benefit to replacing stock frequently?
Anyway, I think I know what's going on. You mentioned them having dry food? A lot of ferrets will choose dry food over fresh meat. It's like a 12 year old choosing McDonald's over vegetables and steak. If they have both available at once and have developed a taste for dry, they will tend to leave t