So sorry for the losses. It is difficult to predict what may be going on without knowing all the specifics. There can be various reasons puppies of this age may die and it's not always mother dog's fault especially in older puppies of this age. Possible causes include inadequate milk production, puppies being unable to nurse due to being too weak, mother dog having mastitis, herpesvirus infection, babesia infection just to name a few. It would be best to have the remaining puppies and mom evaluated by the vet to play it safe as there may be something going on that needs addressed.