Oldest rat had a bad litter...suggestions?

[Gorelith]

My oldest breeder rat is a female, she's given me several great litters over the passed 2 years. Last night she had another long anticipated litter although she only had 6 pups, 3 of them were D.O.A. She wasn't line bred, she was hand selected from premium stock, I always gave her a minimum of 1-2 months recovery time (without a male present) after every litter. Is she just getting old and passed her prime? Is it time for her to become a feeder and not a breeder? She's alone now with her 3 live pups. I've never had D.O.A. rats or mice before, did I do something wrong or is she just getting old? Any and all help would be appreciated.