Switch to starter crumbles, and add about 100 mg niacin to each gallon of water for the ducklings. That should help. Also, give poultry vitamins once a week or two to help cover for vitamin deficiency.
Some ducks are more sensitive than others to missing nutrients.
Use niacin capsules, not time release or no-flush, just plain capsules and open them and pour the powder into the water.
My five week old duckling has been laying down most of the time for the last two days. She waddles along on her belly to the food/ water bowls and soils where she lays. She does get up and stretch and I've checked her legs there doesn't seem to be any damage and she doesn't react in pain. But when she walks now she seems a bit unsteady.. Is this normal like growing pains, or should I be worried?