Give them some toilet paper tubes to play and hide in. I give each of my colonies a big handful of shreaded paper. They burrow in it and hide in it and in general really enjoy it. The point is to make them feel secure. Hope I gave you some ideas.
Mice can be the most frustrating animal on earth.
I had a colony with one male and five females. They had been together for about a month when one morning one of the females was not only dead, but her head was missing. Next morning another female was decappatated. I introduced the remaining three to one of my Jungle Carpet Pythons. Try a new source for mice.