I can't offer more suggestions, just sympathy. I've hatched lots of chicks, my own and some others, and lots of shipped hatchery or feed store chicks. Over the years I've had some batches that just make a hobby of up and dying. Exact same brooder set up that dozens of other chicks have thrived with, some batches just die. I do believe sometimes it's either the parent stock or just unthrifty chicks. It stinks