Its strange that you haven't been able to get any rat pups to survive. If I were you, I would hand-pick a group of female feeder rats, raise them up to size, and keep them seperate from your pet rats. I don't think anything is wrong with your colony, its just that maybe your pet rats' daily routine isn't conducive to breeding and rearing offspring. Pets should be pets and breeders should be breeders...

I wouldn't abandon the rat idea completely. Once you get a group of mice going and see how bad the odor is, you'll wish you had gone with rats.