She could have been bullied by one of the others hence the not eating & general depression of her general health , leave her on her own untill she is back to normal
What i would do is try her again with the others once she is fit & simply observe the behaviour , she just may be a ferret who likes a quite life & isnt really suited to the rough & tumble of living in a group
dont make the mistake i made after i got my first ferret , i was told ferrets have to have other ferret company so i did as was advised & got a second ferret, problem was being so new to ferrets i