Mottled houdan

Are mottled houdans the most fragile chicks? I have the hardest time trying to keep them alive. I’ve had 2 different batches and all 7 die out of no where when all the rest of the other kinds of chicks live. They have vitamin/ electrolyte water, brooder plate, good kalmblach food, grit. I have polish, wyandottes, Japanese bantam, silkie, pearl onyx, blue leg beauty. And I have the worst time getting my houdan babies to live.