you know, that doesn't look neurological to me, it looks like muscle weakness... especially the quivering in the legs of the white crested duck and the panting in the first one after it went down. if they're getting fatigued, a brief rest may be what they need to get back up again and look normal, at least for a while. if they're getting that tired and showing it that fast, I'd expect it to be progressive ... that is, they're getting weaker over time... so you'll see more episodes. the first duck did show slowing down before losing it, like it was getting exhaused.
how is their muscling? do they have meat in their thighs and chest? or is their breast bone prominent? you'll need to pick them up to check, feathers can hide a lot of muscle loss. you shouldn't feel the breast bone sticking out like a ridge if they're normally muscled. (you still may find it, but it should feel like there's meat there around it, not like an alread-picked over roast chicken.)
if they're thin its probably nutritional, but hard to say without more info if it's quantity of feed, quality of feed, or absorbtion. do they have free access to feed all the time? I saw something similar on a friend's ducks (first time duck owner)... they'd been feeding scratch only, and not enough of it, although they had full access to nice pasture with bugs and grass... problem cleared up when they went to all-you-can-eat game bird feed, scratch, and oyster shell in separate pans (let the ducks choose).
I'd be inclined to try the niacin, maybe switch to a higher protien feed, and perhaps add some probiotics as well to make sure they're getting good utilization of what they're eating. maybe pen them for a few days to limit energy expenditure, make sure they have 24/7 feed availability, and feed cut greens, grass or what they seem to like. that'd let you see how much feed they're actually consuming. If they're very mobile ducks, some of the weaker ones may be spending more time catching up with the others than eating, or may be getting bumped out of the feed trough and not getting their fair share. if a week or so of controlled environment isn't showing any improvement, or shows continuing loss of ground, I might worm them, in case you're feeding parasites instead of ducks. I've never has an issue with worms, so I don't know what's appropriate, but someone on here probably has good suggestions.
anyway, just my thoughts on what I'd try.