What breed is she? Did she have others that did not make it? How is the mom acting? Is she eating and active? Are you 100% sure that she has no retained dead puppies? They normally do not want to care for a pup if they them self may be ill or if they know that something is wrong with the puppy. The generally know when something is wrong. Or if she is young and this is the first one she may just be inexperienced. I feel you need to take them both to the vet for an exam to rule out any possible problems. You may end up bottle feeding the pup.