You need to give the apple cider vinagar a few days.
If you have chickens under 6 month, they need to be on medicated feed. If they are older than 6 months, you can buy Sulmet, Amprolium, for cocci. It is popular for bloody stools, but it doesn't have to be bloody for them to be feeling...
If you have changed food or given them something that they usually don't get, that can cause it. I would also treat for Coccidiosis for a couple of weeks. Try the apple cider vinagar first and then go to the other.
I use the vinagar when they are babies and it works like a charm.