Mice can be as easy as rats if done right that implies a few things

Food meeting their needs you need to provide 16% 18% of protein ideally.

Fresh water at all time.

Limited stress (they really don't need to be mess with more than once or twice a week (cleaning and "harvesting"). I interact with mine once a week during cleaning at which time I also pull out my feeders.

Not being over crowded

Now keep in mind that they are scavengers as well so if their babies die they will dispose of them, also keep in mind that if they are not capable of nursing the babies they will cull them as well.

Now I doubt food is the issue or the bedding, it is very possible that you have bad females it happens and the only solution is to start from scratch again.

Years ago when I first started with mice I had to try 3 different times before getting a good colony started, this mean culling any animal that were cannibals, chewers, biters, since than I have not had a single incident of cannibalism in my colony.

Here is how I run things I have 2 females per tubs and males runs through 3 tubs each staying with the females 2 weeks at the time. I have found this method to work best for me compare to harems.