Where did you get your mice? Starting with good stock is a key factor in success.
Were they provided with all the necessary food, water, and bedding? Was it a first litter?
To me, 12 is a large litter size for a mouse to handle. I usually cull mine down to 6 maximum. This results in less strain on the mother, and healthier, fatter babies. That might not be the best for a person breeding specifically for feeders, but it's something to think about.
Also, did you separate the male and female after the litter was born? You may want to try that also. The female can be bred immediately after littering, and this causes a lot of stress. The male will chase her around, sometimes while she is delivering, trying to mate. IF the female becomes pregnant, then she is also trying to have her body provide for 2 entire litters at one time, which is typically too much stress on the female, to produce healthy babies.
If the babies are not healthy, the mother would rather kill them off, than strain her own body, and risk her life just to take care of babies that may not make it.
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