It is so tough to begin to figure out what is causing ducks to have trouble. They often don't show signs till they are really struggling.
That said, there are some things you can try. Vitamins is a good start. There are poultry vitamins (I like the ones that include probiotics and electrolytes) available at feed stores and online. I think it would be good to give her some. In the meantime, some probiotics can be given to her by adding a tablespoon of plain unsweetened yogurt to a cup of water. Some ducks don't like it, some do. Worth a try.
You can give her electrolytes by giving her some pedialyte. Check the topics here on BYC to see if it needs to be diluted. It's best not to overdo it.
One of my ducks, Sieben, is not the most robust runner in the flock. She hates the cold, she isn't too keen on heat, she lays big eggs but has had trouble getting enough calcium. When that was at its worst, she would plod, and labor, and breathe heavily. Poor girl.
I took her to the vet because I had recently lost Neun and just couldn't bear to lose another, not so soon.
Anyway. sometimes it can be a nutrient deficiency (not your fault - ducks seem to vary quite a bit with what they need nutrition wise), so vitamins can help.
It can also be an infection, which is scarier. Those require antibiotics. We have not had to give antibiotics yet, so I cannot tell you more than if you can have someone look at the duck and recommend antibiotics (or not), and if so which kind and at what dosage.
Many people make the call on their own and there are topics about how much to give and how, here on BYC.
Another thing people give their ducks as a tonic is apple cider vinegar. I add a few teaspoons per gallon of water.
If the duck is eggbound, you may be able to feel the egg (careful - don't want to break it - big problem, that) or see that she has an egg belly. If so, I am told that often a nice warm bath for at least half an hour can help relax her muscles so she can lay.
Those are my thoughts - I hope there is something helpful in there.
