I just realized this is the same OP who before was asking in another thread why their ducklings were always hungry, and said they couldn't get a duck or chick starter. So I suspect they are not being fed a good diet.
@duckduckchickenrun what country are you in? You really need to get your ducklings a proper feed, and if people are able to breed and sell ducks in your country, people must also be able to get proper food for them. I suggest you ask the person you bought these ducklings from at the market what they feed their ducks, and start feeding it to these ducklings.
Have you stopped keeping them in a bucket and moved them to a proper brooder? Or moved them outside?
If you're not feeding them the right stuff, it's very possible that this duckling isn't eating because you're not offering it the correct food to eat. You really need to get a proper food for them.