The thing with coccidiosis is that there are different strains in different areas and whilst chicks may have acclimatised to the cocci in their area, as soon as you move them, they are vulnerable to new strains and coccidiosis can be a fast killer.
Medicated feed contains an anticoccidiostat.... a medicine that helps the chicks cope with coccidia whilst their body builds up resistance to it. It is not an antibiotic or hormone or anything like that and chicks that are incubator hatched and reared need that medication particularly if they are moved to a new home. Chicks that are raised by a broody hen pick up immunity from the hen via her poop and are exposed to coccidia from day one, so their body learns to deal with it without the need for medication, but hatchery chicks come from a fairly sterile hatching and brooder environment and are therefore more vulnerable when they are eventually exposed to the organism in the environment.
It was probably a mistake not to give them medicated feed for this reason and now you are having to give a stronger dose of the medication ie Corid.(amprolium) which as KikisGirls explained, acts to block thiamine from being absorbed, resulting in the coccidia being unable to thrive in the chicks gut. Amprolium(Corid) is considered a medicine as far as I am aware..... hence "medicated feed" It's just that many people assume "medicated" means antibiotics or hormones and it doesn't.
I hope the Corid works fast enough to save the other chicks. It's a good idea to follow the treatment up with a vitamin supplement like Save a Chick or Nutri Drench once you have finished using the Corid.