Hi,
Which breed is the slow one?
A couple of suggestions -
Check to see if she is looking bloated compared to the rest, if you catch her again have a feel of her tummy. Campbells are susceptible to peritonitis, a consequence of laying so many eggs. Egg yolks can miss the tube and gather in the belly causing infection.
Is she moulting? Our girls get very tired when they moult, especially if they have not had the good sense to stop laying for the process.
I give them chopped up garlic in their food as a natural antibiotic so I would do that as a matter of course, pop some cider vinegar in their water and keep an eye on her. If she is moulting you can get some conditioning powder for their feed.
Also - if you have not done so already you may want to try the Poultry Keepers Forum, there's a few people on their who have a lot of experience with ducks.
Hope she's better soon.