My 4 week old puppy looks weak and wont drink it's mothers milk, what's wrong with her?

She looks very week and when I try to make herdrink the mother's milk she won't drink it. my dog had 3 puppies and this one is sort of the runt of the litter. The other two puppies look fine and this one looks weak. and while the other two puppies try to play with this one, she just let's them jump all over her while she just sits there and does nothing. She feels very boney too, where my whole hand can wrap around her waist. She also acts unnormal. she sleeps on the concrete instead of her igloo where the rest of the puppies are, and when I try to put her back in, she goes out and sleeps on the concrete again. I noticed that she likes to be alone in a little corner and she tries to hide. She also cries every night and I don't know why. What do you think is wrong with her?