
It's good the puppy has been given antibiotics because milk aspiration can lead to pneumonia although if your vet has checked and her chest is clear, that's all good at the moment. Clearly something isn't right with this puppy and there has to be a reason, usually cleft palate, for what's going on. I think I certainly would take this puppy back because there may still be a very small cleft which is causing all this.
We had one, of an original 9, 4 lost after a C.Section was eventually done because they were too weak to survive by then, who was bottled by the staff before we got in to collect mum and her then 5 surviving puppies. Stupidly they were bottling them when in fact all they needed at that stage was warmth and to be able to gather their strength after their shocking (and it was shocking) start in life. By the time we got them home, milk was coming from his nose and despite all our efforts, he didn't survive.
Just to add if she's a singleton puppy now, perhaps mum is producing more milk than she can easily handle and is perhaps taking in too much, too fast? And I'd be worried if her only sibling was DOA.
Hoping you pull this little one through. Breeding can be so hard at times.