There is something called Brooder Pneumonia and Duck Plague that I think are causes of high mortality in Call ducklings.
No symptoms other than lethargy, Sonja.
There are times when smaller ducklings are rooted out of the feed and water by larger ones.
Something else I have heard is that when Call babies are confined to a small space-you will have some that "volunteer" to die in order to make room for the others. They are a very odd little species and that's the reason that they are SO hard to hatch/raise/breed.
What are you feeding them?