The head/neck position the duckling is exhibiting is known as Torticollis (wry neck) which can be common in ducklings; determining the underlying cause of the wry neck can be difficult since there are many etiologies that can cause the wry neck to occur. In ducklings, I would be considering bacterial infections, vitamin deficits, genetic defects, electrolyte imbalances, and neurological damage - which can be more likely to occur if the birds were shipped and handled roughly during that time.
While we likely can't effectively target each one of these conditions, we can offer a broad range of supportive care that may help improve its condition. What does basic supportive care include (?) All basic needs must be addressed, ensure the duckling is eating, and drinking adequately enough - if needed mash some scrambled eggs, and offer it directly to the duckling. If the duckling is getting bullied consider moving it to a pen where it can see its flock mates, but can't physically touch.
Watch the duckling for signs of chilling, or overheating, change the temperature as needed to cater to its current state. To address any vitamin deficits, more specifically thiamine, and vitamin E, consider administering a broad range poultry vitamin such as Nutri-Dench, or Sav-A-Chick, and hopefully, you'll see some additional improvement.