Does anyone have any idea what is wrong?

[Homie Four]

I hatched some chicks around the first week of May and they've been doing great until now.
I noticed three of my chicks were acting very lethargic, dragging their wings on the ground, barely walking, but they are eating a drinking normally.
I moved these three away from the others and put apple cider vinegar in their water and the others water just in case.

One chick passed away yesterday, and another I think is gonna pass away today.
She is keeps scratching one spot on her head and flipping over.
She also feels very underweight even though I am watching her eat.

The last chick is acting perfectly fine though now, much better than before, moving around great.
I see nothing wrong with their stool. These chicks had coccidiosis near the end of May, but I took care of it and they were doing much better.
I've checked them many many times for any kind of bugs, and I can't find anything.
I'm very unsure what to do.

Any ideas?
Thanks

 

[Eggcessive]

Were they vaccinated for Mareks disease? Coccidiosis can recur again after treatment in chicks who have not built up tolerance to cocci in the soil. You won't be able to know for sure what the chicks are dying of unless you get one necropsied by either your state vet or nearby poultry college lab. Refrigerate the body and send or take it in right away. You might want to give another 5 day treatment of Corid in their water, just in case of cocci.

 

[Homie Four]

Were they vaccinated for Mareks disease? Coccidiosis can recur again after treatment in chicks who have not built up tolerance to cocci in the soil. You won't be able to know for sure what the chicks are dying of unless you get one necropsied by either your state vet or nearby poultry college lab. Refrigerate the body and send or take it in right away. You might want to give another 5 day treatment of Corid in their water, just in case of cocci.

No, they were not vaccinated for Mareks, so that's what really scares me. I was thinking cocci could've came back, but there were no signs of anything wrong in the stool. Yeah I was hoping to buy some more Corid tomorrow since I don't know what else to do. I don't have the other chick's body anymore, but if this chick ends up not making it then I'll probably look into sending it in.
Thanks

 

[Homie Four]

So it's easier to understand, the Splash chick is the one that seems very sick and the Mille Fleur is the chick that is much better.
Update: The Splash chick is doing a little better, I'm very surprised. I ended up putting some sugar in their water yesterday. I still have no idea what's wrong though, the only sign of her not feeling well is being droopy and lethargic. Even though the mille fleur chick is 100% better, I'm gonna keep him with the splash that is still sick just in case something is still wrong, and/or taking him away can make the chick depressed.

 

[Eggcessive]

You may want to put some vitamins and electrolytes in their water for a few days to see if it helps them perk up more. Good luck.

 

[Homie Four]

You may want to put some vitamins and electrolytes in their water for a few days to see if it helps them perk up more. Good luck.

Funny you say that because I was just at Wilco an hour ago and picked some up. I have no idea what was wrong with the chick, but she seems to be doing better though. Keeping my fingers crossed that she makes it.