Yes, I have. I don't know the exact cause, it could be nutritional or perhaps genetic. Assuming you are using proper food, it is perhaps just a genetic thing.

My sister, MANY years ago, when she was a teenager, found an abandoned wild mallard duckling in its nest after the mother and brood had left. Same thing, it couldn't walk, but crawled. She brought it home and put it in a box with a light. The legs stuck out straight back behind it.

Despite the fact it never could walk, it lived 4 or 5 years. She built it a pen. It had a small tub, couldn't really swim but could float and bob. It was very friendly and tame and seemed genuinely attached to her. It actually had a very good life. It was even quite clean, just had to make sure to keep the straw and bedding clean and fluffed so it didn't sit in its waste.

i am assuming that we are feeding corectly as all our other ducklings have done just fine. her elboows are begining to stick up from her back but she still manages to belly crawl to food and water. she will have a wonderful home in our house and not the barn for as long as she wants to be with us. she truelly is a special little duckling. thank you for your help

Try giving it Vitamin E; use a gel cap and poke a small hole in one end. Pry open it's beak, and squeeze the contents inside, discard the empty shell. Do this twice a day for three days and if it's a nutritional deficiency, it will improve.