I don't know if this would help. I purchased 6 chicks from a local chicken farm some times ago, 2 silkies, 2EE, 1 Barred rock, and 1 polish. 2 Silkies were fine the first couple weeks, but then, at 3 weeks old, they suddenly became droggy and died 1 or 2 days later, then the polish had similar sympton and also died a couple days later. I went back to the farm and ask for replacement. They told me that silkies and polish are fancy breeds, unlike other comercial/common breeds (RIR, ISA brown, Barred rock, EE..etc), their immnue systems are not as good. I need to switch to medicated food to prevent them from illness until they are 8-10 weeks old. So, I did, my silkies are 9 weeks old now and still fine. I don't know if your other chicks are consider as fansy/uncommon breeds, I though it may be a good idea to feed them medicated food for some time.