You need to consider the quality of the feed you are using too. Please tell me it isn't Dumor brand, or some
Tractor Supply garbage.
You also need to consider that it takes about 2 days for an oocyst to sporulate, and only three days after ingestion for a cocci protozoan to show symptoms in a chick. Are you planning on using Amprolium in the water or are you using medicated feed. I had some chicks become very sick on medicated feed once. When I changed feed, gave them supplements, then used Corid in the water, I was able to get them back on track.
Also, does the chick walk without stumbling? Does the chick appear "knock kneed" when standing? Are the toes nice and straight, or crooked or curled?
Each chick has its own immune system that will be strong or weak. Some runts can grow into vigorous chickens too. I've seen it happen myself. You just need to try a few things that may give it some support.
Start using Probios soluble powder in the water 3 days a week. At the same time offer chick scratch (finely ground corn, red wheat, and milo) in addition to the starter feed. You can usually get
chick scratch at your local feed store. That is what I do for the first week when pasting up commonly happens. Make sure the temp is somewhere around 85 degrees and they have enough room to move away from heat if they choose.