All the other pups have died, her last litter had the same fate.

It's time to talk to your vet and get the remaining puppy on a puppy formula.

I would also make an appointment to have the mother spayed, it's time to stop breeding her.

Good luck.

Agreed, obviously something is going wrong here for 2 whole litters to have died, poor mum and pups.
Time to close the puppy factory.

Get some milk replacement formula from the pet store or your vets, soy milk is not a suitable substitute for a young puppy.