A) Make sure they have things to chew, plenty of water, and especially a quiet, hidden spot for the enclosure. Mice are really nervous critters.

B) Give them a good mixed diet including lab block and senior dog food to ensure good protein levels. Becky's Rodent Mix here on the forums is a great feed!

C) You don't have to separate the male and female. In fact, sometimes being moved around a lot can cause the losses. Mice don't like disturbances unless they are really used to you and trust you.

In the end, as was said, if the mom just keeps doing it despite precautions then you should replace her. Good luck.