I also have a couple of ducks who seem to have trouble with shells.
I have read a few things on the topic, and some of my ducks have responded well to calcium gluconate, 1 teaspoon per half cup of water added to feed, or a quarter teaspoon in a couple of tablespoons of mashed peas once a day.
Lately I am trying calcium citrate, 500 mg per day for 10 ducks.
I think there may be more to it than calcium intake, and I know there needs to be a proper calcium to phosphorus balance, as well as enough vitamin D.
There's oyster shell free choice, cleaned crushed egg shells, and other calcium supplements to try.
One person has suggested to me that some vitamin C might help.