Try cotton balls with warm water on them. Try to stimulate the stomach in gentle circular motions and stimulate the anal area the same, lift the tails up.
When they pee on the cotton calls, take that and rub it on Mom's front legs and hold it up to her face. You can try doing the same with some of the puppy poo. She will most likely lick her legs. Don't force it but pick a puppy up and hold it near her nose/face for her to smell, assuming she is not baring her teeth. Is she a first time mom? Nay take a day or a few but this usually works.