She could have an infection, or a nutrient deficiency, a number of things. Majestic Waterfowl published this list of possible reasons for symptoms and for wings drooping I found this
[TR] [TD]Drooping wings[/TD] [TD]Pneumonia, [COLOR=2B6C24]
Aspergillus[/COLOR], [COLOR=800000][COLOR=2B6C24]
Aspergillus II[/COLOR],
Lead poisoning[/COLOR], [COLOR=2B6C24]
Zinc poisoning[/COLOR], [COLOR=2B6C24]
Zinc Poisoning II[/COLOR], Cancer, [COLOR=2B6C24]
Poison consumption[/COLOR], [COLOR=2B6C24]
Poison Consumption II[/COLOR][/TD] [/TR]
If you can get a good duck vet's help that would be best. For poisons, sometimes food grade activated charcoal slurry can help.
https://www.backyardchickens.com/t/700526/flushes-for-aiding-in-toxin-removal#post_9508213 Certainly, a good diet, clean water, and healthy environment will help her. She may simply be run down.