Weak legs Runner duckling 4 weeks old

[Moments]

Hi folks! Thanks for your help. I hatched 7 Trout Indian Runner ducklings. I sold 4 and kept 3. Sadly, one I sold passed away and then the buyer contacted me that her other three seemed to be having issues walking. Having raised runner ducklings before I know they can be a bit wobbly to start but one of hers was hardly walking at all though all three had issues. I have been giving mine nutritional yeast and unmedicated chick starter (Hi-Pro) and they didn't have issues. She was giving hers brewers yeast and a similar starter feed just a different brand (Country Junction). But they were in a small cage with food and water immediately beside them so I figured they just hadn't had motivation or space to move. I offered to take the ducklings back and rehab them. I've had all 6 for a week now, I have not observed any deformities or twisted limbs, etc. Two of the three have recovered well, a bit more wobbly than mine but visibly improved. But one, the biggest one, is persistently unwilling to walk unless prodded. He walks well, but tires quickly and will often refuse to move after a short time. See attached video. Even swimming he'll just float and not do much paddling.

I swim them at least twice a day in a big tote and they have access to a pool now during supervised outdoor time (usually 2-3 hrs in a larger space) which they can get in and out of themselves and it's shallow enough they can stand in it. They sleep in an 8'x6' space which I have been keeping food and water in at opposite sides to encourage movement (normally I don't keep feed/water in the coop).

Are there any other tricks I could employ to get this duckling moving more? Could it be neurological? He is definitely shaking when walking so I am leaning towards it being a weakness thing but I notice he often steps on his own feet and trips like he is very uncoordinated. He also seems to stand on his "knees" rather than his feet though when made to walk he uses his feet fine.

Normally I would start mixing starter and grower feed for week 5 but I'm concerned about him putting on weight too quickly without building strength. Thoughts?

My runner gal was showing signs of weakness in the legs very early on. She could walk, run if she wanted but only in short bursts and would lay down while the others were all over the place. She sometimes would stand to eat at first but then lay down to finish. I started giving her brewers yeast once I noticed the problem (eggs fresh daily brand, there's some extras added in with the brewers yeast for added benefits). As she started getting bigger I noticed her legs shaking while standing. She'd seem to be getting better but then not so much. I started doing Bragg's brand nutritional yeast along with it, sometimes alternating or doing half and half and she improved. Also scrambled eggs daily with a little baked sweet potato mixed in, sweet potatoes are a natural anti inflammatory. Daily swims in the tub to build strength in her legs. She's almost fully feathered now and her legs are doing fantastic! I was pretty concerned for awhile but she's all good!