If the loss of down is in the category of "normal", you should be able to see tiny stubs of new feather growth pushing through the skin under the thinning down. These pin feathers will cause down to fall out as they grow in.
If you see large areas of naked skin without these new pin feathers pushing through, you need to figure out why. It could be parasites. Examine the skin closely in the area near the vent for tiny, moving red "dots", indicating lice. Watch for lots of scratching going on.
Watch the behavior of the chicks for signs of an individual plucking down from its neighbors' bodies. This is a not uncommon problem. You might be able to solve it by disciplining the culprit with a gentle poke on the back each time you see it nibbling on a mate.
Sometimes, though rarely, there will be an underlying pathogen at work. Watch for lethargy, watery poop, and other signs of sick chicks. It might also be a genetic defect.