It could be a different breed, it could have a nutritional deficiency, parasites, or just be one of those ducklings who struggle to grow.
Elfie was two weeks behind in development for her first six months and now she has caught up and doing fine.
They need good food, occasional vitamins and probiotics especially during times of stress, fresh water, the ability to wash off in fresh water frequently. Of course they need safety, and proper temperatures.