Baby multi death??

Hi, I recently aquired a colony of Multimammates for breeding and so far they have produced 3 litters, the oldest of which would be 5 weeks now and a litter of 9, yesterday however I found 4 of the babies dead in the cage and this morning found the remaining 5 dead. They all seemed healthy and I had witnessed them all eating solid foods and some of them drinking from the water bottle. I had suspicions that they may be dehydrated as their skin was tenting slightly when scruffing them so I put in a shallow bowl of water as well as their usual water bottle (which, as I said, I had seen at least 4 of the babies drinking from). I had given them a small bit of apple (no skin or pips).

I am going to speak to my vet later to see if he can help, I'd hate for the same to happen to the other 2 litters which seem incredibly helthy at the moment - the oldest of which have just opened their eyes. But in the meantime, can any one help or suggest anything which may have caused this? Was it the apple? Or possible dehydration? If it was dehydration, what can i do to help the other litters from having the same problem?