I think I'd consider having a necropsy done on one of the chicks or younger birds that dies. Perhaps your adult birds are carriers of something (like CRD/GM, etc) and much older pullets and birds can usually deal with these infections much better...with a much lower mortality rate. The younger birds have a much higher mortality rate from them. They could be hatching infected (passed from a carrier hen) or being contaminated after hatch by the birds-- or transferred by you 'tracking it in' to them if they're in a different brooder area.
A necropsy might be what's needed to know what you're dealing with. I've read that some some state vets/department of ag will do a necropsy on a potential problem in a flock for free, or a greatly reduced rate. I'd check your area and see what's available if that's an option at all. (if any of your birds have ever had a "cold"-- coughing, sneezing, nasal discharge of any sort...even if seemingly mild... you ARE probably dealing with CRD of some sort)
Otherwise, definitely check for lice/mites. Super tiny little buggers and sometimes hard to see (IMO), but they can take a small bird down rather quickly.
If it is some infection/disease that's being contracted by them, antibiotics will only help IF it's the 'right one' for the job-- for the problem you're facing. Not all things are sensitive to the same antibiotics.
I'm assuming some you raise make it to adulthood? Not all would succumb to many of the illnesses out there...but they usually do have a higher mortality rate in chicks and younger birds.
