If the ones who acted sick had coccidia, then it's likely the others who died, (who didn't act sick) also had it. In my experience, some chickens act sick when they coccidia, and some don't. If they were already dying from coccidia, then they will usually continue to die from it, even after you feed them meds, until they have enough meds in their system to counter the coccidia. But unless you've seen blood in their poop, it's not likely that it is coccidia. Coccida causes them to have VERY bloody poop. It's almost impossible to miss. Coccidia also would cause them to stop growing, I don't think. That being said, I have no idea what it might be. I looked it up and have found nothing. If you were feeding them cracked corn and bread or something crazy like that (don't laugh, I've seen people ask why their chicks weren't growing while they were feeding them those things and then be shocked when they were told they needed to be feeding them chicken food...) it would make sense. I'm afraid I can't be much help to you. :\ Sorry. I hope you figure out what's causing it. I'll look it up some more in a bit.