My thoughts are that it is something in your space, whether being brought in from the main coop or in the box or it just is. When the little ones get sick, they are unable to fight off the cocci and also the bigger chickens will have issues with cocci when they are sick as well. Are you using medicated feed? I really hate that word medicated when it comes to chick starter as it gives the wrong impression. If a chick is sick, it won't be eating as much and so even the feed won't work against cocci.
Virus aka cold, antibiotics won't help unless there is an underlying bacterial infection, treat symptoms for support
Bacterial - antibiotics will help, usually needed asap
Cocci- treat with Corrid or such
Once you do the antibiotics, treat with probiotics to help get their system back.
It seems that you did the right thing in giving the amprolium It gave the chick a chance to fight off whatever is going on without dealing with cocci on top of it. Good luck, sounds like you are doing everything right.
Virus aka cold, antibiotics won't help unless there is an underlying bacterial infection, treat symptoms for support
Bacterial - antibiotics will help, usually needed asap
Cocci- treat with Corrid or such
Once you do the antibiotics, treat with probiotics to help get their system back.
It seems that you did the right thing in giving the amprolium It gave the chick a chance to fight off whatever is going on without dealing with cocci on top of it. Good luck, sounds like you are doing everything right.