One of our mallard ducklings seems to have been hatched with a problem... He tends to jerk his head back over his back (so his head is upside down) and is very uncoordinated and spastic. He eats and drinks fine if you hold him and hold his head straight. Been giving him a nutrient drop for hatchlings just in case it might help him out.
Usually when I see that they don't make it. We see it in hatchery ducklings at the feed store, sometimes they get too cold or get hurt in shipment or there is a fast-acting disease that can cause it too. I just keep them comfortable and help them out until it's clear if they're going to get better or not.