Alan
A friend of mine, well known breeder and trainer of gundogs up here in the Scottish Borders, used to give his lactating bitches a supplement with her food that was purchased from the vets, SA37.
I have since always given it to any bitch of mine with pups and in turn sprinkle the powder form of this additive onto the pups food when they move onto solids. By bitches and their pups have always thrived. May be worth a go.