How old were these weanlings? Were they a little runted? In my experience (I work in a research facility where I deal with thousands of mice a day), the number one killer of otherwise normal mice tends to be dehydration. If this is the problem, make sure they can reach the water source. Also, if you feed them "lab block" type food, soak it in water for 10-15minutes, and then feed it to them.

I hope this helps!