That is almost exactly what I went through with mine last year. I thought I did something wrong. At least I wasn't alone with this problem. It is not fun to go through.My mama ducks hatched out babies last year and in 2 batches hatched at different times, I had a similiar experience. Everyone was fine, eating & drinking (I added vitamins to the water), then at about day 5 - one would just start to go weak and not be with the others. In both cases, I brought in the duckling and tried my best to save them but they eventually passed. No idea what the problem was since all the others continued to thrive and are now healthy ducks.
I hope yours makes it though, it's so hearbreaking to see one of them not make it.
Hope we can find a way to save your little one...