[souffle]

You need to consult a vet at least or a dog forum for knowledgeable advice. It sounds as if mother knows there is something wrong with this puppy. If you can hold her to the mother and try and get her to suckle this would be far better than hand rearing. It is a 24hr a day job and needs you to stimulate the pup to wee and poo (mother would lick her to do this then clean her up) She will need very frequent feeding with a substitute puppy milk like Lactol which your vet should stock. Warmth and contact will also be essential. I would try my utmost to get her back with the mother as she stands the best chance there and seek vet help in case she is ill.