Okay, I'm still not clear on her age . . . (probably my brain - slows way down after sunset).
Anyway, slight possibility she needs more niacin than the others. Some ducks seem to be sensitive to niacin deficiency. If she's still pretty young, and you're feeding poultry layer mash, she may not be getting what she needs. It's not for sure certain, just a possibility. Also I'm told some non-laying ducks and younger ducks (as well as drakes) are sensitive to too much calcium in the diet, so switching her off to a grower/maintenance ration may help.
Sometimes ducks also have internal problems that do not become apparent until they're mature. I think my Drei may have been like that. She grew up okay, but died at about seven months of age. We didn't do a necropsy, but that's water under the bridge.
If you have a vet, it may be time to take her in. But if that's not an option, I always go with - fresh water, quiet warm safe place, lukewarm bath time, vitamins/electrolytes/probiotics, a few peas to check appetite, sometimes activated charcoal (can be bought over the counter in drugstores - it binds to toxins, I am told). I sing and talk to my ducks - I think they respond well to tender sounds.
I'll be praying for you both to get better soon.