Her digestive system may not be developed to the point of accepting solid food. A baby's main nutrition is gained from breast milk or formula until the age of one anyway and so I wouldn't worry. Let her nurse as often as she wants to nurse. If your milk supply is low, which may be why she wants to nurse so often, you can supplement with a non-dairy formula.
My granddaughter was allergic to the protein in milk and didn't take well to breast feeding. She thrived on Alumentium, a non-dairy, easy to digest formula.
Also, babies do plateau in their weight gains and height. If her doctor is not concerned about her weight, I'd also stop being concerned.
Perhaps a lactation specialist could help you figure out if she's getting enough milk.