Unfortunately it is very difficult to give you the answer you are looking for without a necropsy of the dead pups. (Dystocia, nutrition, congenital or genetics, environmental). Neonatal pups (pups pre-birth to two weeks of age), most common cause is fetal stress/distress syndrome. Occurs with pups that have had a difficult or prolonged delivery. Typically are oxygen deprived resulting in neonatal illness or fetal death within the first hours to days after birth. Other possible include failure of passive transfer from the mother to mother not having enough milk initially. There are many causes, but without info that a necropsy would provide it’s just guessing. Sorry for your loss.