We just did as Adam was saying and had another rat mother raise both her young and another female's young (totalling 16) and it went perfect. The original female was not eating her young but refused to nurse them or lay with them and literally jumped out of her ten gallon maternity tank the day after they were born and ran off. We recaptured her 3 days later and she's "no long with us".
Maybe some of them are just not great breeders.
~~Jo~~