Are the roosters or cockerels by any chance getting bullied away from feeders and waterers? Depending on your location, a necropsy is affordable, and then you would know what is killing them. Also check the feed or any scratch grains for any moisture or mold. It can appear overnight if a little...
What is your location? How old are the birds dying? Can you post a picture of any or the combs? Most state vets will do an necdopsy/autopsy on 1-2 birds to look for a cause of death. Here is a list of state vets:
https://www.metzerfarms.com/poultry-labs.html