Sick hens, please help!

[Mama Chicks]

We started with 6 baby hens in April of this year, since then 3 have died the same way, the latest one today. They all end up with little muscle tone or control at the end. They can't stand or get knocked over easily, and can't even hold up their wings. The last one was still eating and drinking water last night. I don't understand why this is happening, you would think if they were getting in to something while free ranging or if something was wrong with the food or coop they would all be sick but they are not. I don't know how to stop it once it has started or even what to change. I clean the cage and coop weekly and water and food almost daily. Just started added ACV to thier water today hoping it might help. Any ideas about what might be causing it or what I should change. Hoping someone out there has had a similar experience, don't want to lose any more chickens! Thanks in advance...