weight loss

I have a problem with one of my girls. Basically the weight is dropping
off her to the point where she looks like a scraggy bag of bones.
I have had her to the vet for a checkup and he says there is nothing
obvious wrong with her (no lumps, breathing and teeth all ok), but she
is still loosing weight.
She is in a cage with 5 others (all female) and does not seem to be
getting picked on and I see her eating.
Any ideas?

Cheers
Rockbox