If by "Walmart foods" you mean the generic "Ol' Roy" stuff, there isn't enough nutrition in that stuff for any dog, much less a dog that is trying to produce a litter. I would really recommend switching your dogs to a premium food of some kind, preferably a meat-based kibble with no corn in the first several ingredients.. . You should also consider contacting a veterinary reproductive specialist in your area. They can be hard to find, but your regular vet should hopefully be able to refer you, or can find one for you. The reproductive vet can test your males/females and give you a better idea of what's going on.. . Have you done bloodwork on your females? Sometimes thyroid problems can prevent b*tches from conceiving. The dog just looking "healthy and energetic" doesn't necessarily mean there isn't a problem.