Mine turned into taking a duck in respiratory distress to the vet. I let it go too long because I didn't think anything of it, but she was only about 9 weeks old so it hit her extra hard. Turned out by then she had pneumonia, multiple bacteria strains, a fungal infection and trichonomas. The first vet diagnosis was through fecal test. Long story not so short... a couple of months and many vet visits later we got a healthy duck cleared of pneumonia and all the other secondary issues. While which was all happening one of my other young ducks started sneezing so I had to separate the two and keep them away from the flock. Again, vet saved them.
BUT what I learned is not to let things go too long with animals. Better get them checked out sooner than later. Get a fecal analysis for her if you can. Especially if she's not eating and keeping up with the other like she should.
I'm happy to try and help with whatever I've learned.