d hypothesize (1) something they are eating, rolling
in, being bitten by? Do they go outside? Do they stay
in the laundry room where they might get a little detergent or bleach?
or
(2) a hereditary problem? Did you use the same sire each time? If so, he and your ***** may each have a recessive gene for this--something that wouldn't show up in the adult parents because they are just carriers, but would affect about 1/4 of their pups.