My chicks are being wiped out

[Buremu Sarapio]

2 months ago I removed all my vaccinated chicks which were around 2 to 4 weeks old from their mothers into the brooder & they started getting paralysed, rotating their necks & failed to eat in the end only 16 chicks out of 103 were left
I have been raising 100s of chicks in the same environment without problems
Then recently I hatched more chicks but decided to leave them with their mothers but still they all die in the similar way the others died, I have changed the feed supplier thrice so as to check different qualities, given them a corid,anti biotics, calcium plus vitamin supplements & even let them free range but they all seem to perish.
Please advise

 

[Eggcessive]

What was the average age that the chicks got sick and died? Could your feed have become wet or moldy at some time?

 

[Buremu Sarapio]

What was the average age that the chicks got sick and died? Could your feed have become wet or moldy at some time?

They started dying 3 days into the brooder but now they start dying 5 days after hatching
My feeds are always free from moulds at least that I ensure by throwing away the mouldy ones