Welcome to BYC. Sorry to hear you're having problems with your duckling, hopefully, we can help, but we'll need some more information regarding the duckling, and what it's being kept in.
Where did you purchase these ducklings? Could you post a video of the duckling? To upload a video of the duckling, you'll need to upload it to a video platform like Yotube or Vimeo, then copy/paste the link here. Could you post a few pictures of her poop?
Is she eating and drinking much? What are you feeding them? What are you keeping them in? What's the approximate temperature in their brooder?