Why are my mollies dieing?

I have recently started to raise mollies. I have always had fish but after an ick wipeout and completly cleaning my tank I decided to go with mollies. Well, I got three pregnant ones (unknowingly) and one gave birth. I loved it! So I got two other tanks, one for the main fish, one for babies only and another for the soon to give birth moms (instead of useing the breeder nets or boxes I just put them in here a day or two before expected birth). Anyways four have given birth and not a single one has lived after birth. The longest living mother I have had only lived about two weeks after birth. The babies are fine, and all nitrites, nitrates, amoniam and everything else is fine. I am feeding them but just cant seem to keep the mothers alive after birth. SO, WHAT CAN I DO? I get attached to my pets easily and want to keep them alive.