HollowOfWisps
Previously AstroDuck
I agree and good point in the vitamin D in addition. I keep infant liquid D3 in my first aid box for the same reason. It's cheap and you only need 1-2 little drops.I would put the oyster shell in a separate container on the side not in with the feed. It's not recommended to add it to the feed. Also, sometimes it's a problem with calcium absorption. Vitamin D helps with calcium absorption. I also add crushed citracal (a calcium supplement with vitamin d found at the store) to their oyster shell container especiallywhen soft shelled eggs are laid. But, I would get her some calcium gluconate and make sure she gets dosed to help get back on track. You don't want her to continue laying soft shelled eggs.
How long have your ducks been laying? It could just be a blip with a new layer, but if it continues you will want to do more than just oyster shell. Definitely get the oyster shell in a separate container so that they can take what they need.