Do you change the padding in a short period after fertility? If so, the mother may recongnize foreign smell on the babies, and treat the offsprings nolonger as its babies.
I have experienced the same problem. When we keep the padding at least 7~10 days after the fertility, most of the offsprings are saved. If you have to change the padding, then use new gloves.
Greetings! This may be due to lack of iron, food stress or emotional stress. Revise the diet of mixed fodders, lighting and the presence of smells of chemicals (for example, when cleaning cages or filler), electromagnetic radiation from devices (hum, noise, etc.). Usually, mothers eat a placenta to replenish hemoglobin. Be a good obstetrician!