Well, hopefully you will get the expert advice of LaurelGrove. She is a member here and knows EVERYTHING about food.
But, until then...
Have you tried a different food? (I mean, Purina One, Purina and Nutro, are they all you've tried?) Growing large-breed pups DO have specific protein requirements. They can't have more than a certain amount, or it will cause their mass to grow faster than their skeleton can keep up...causing problems later down the road. I think its like, 25%, but I could be WAY off on that.
Basically, anything that you can buy at the grocery store is something you should avoid. Especially with a large or giant breed like a Saint. I would also contact your breeder to see if she has any ideas.
Have they been wormed?