Is this mom's first litter? How did the babies look? Did they have any milk in them (white spot on the side of the belly) or were they emaciated? What kind of food are you feeding them?
Some moms just aren't good mothers or are bad milk producers. I have one like this, she would give birth and leave them. Her first litter died, half of the second litter had to be taken care of by another mom that had a small litter, the rest of them died. Now, as far as I can tell, she has gotten it together and is nursing.
If they are not getting high quality food and fresh vegetables at least on occasion, they may not be getting everything they need to produce milk for the babies, as well as sustaining herself.