Breed? Do you have any poultry vitamins? Are you giving your ducklings any form of niacin?
It might be a good idea to put this duckling into its own brooder if you can fix it so it can stay in the same one but separate that way to can monitor its eating drinking and pooping.
Can you tell if it's pooping? A member on here had a duckling that wasn't doing well and found it had pasty butt which will kill them if not taken care of. The bottom needs to be checked to make sure there isn't a blockage of dried poop making it difficult to poop. Usually a nice warm bath will help loosen any dried poop and helps to get them pooping.
There are good poultry vitamins out there too Poultry nutri drench is one. Also very important that ducklings have a good supply of niacin in their diets for the first 10-12 weeks of life to help them have strong legs. Making sure bedding is as dry as possible is another thing, Aspergillosis is another problem that wet nasty poopy bedding can cause. So keeping the brooder clean is important. This is a fungus and causes breathing problems. It's also called brooder pneumonia.