Puppy not gaining weight?

I recently adopted two border collie puppies. One is growing just fine but the other one, the female is very small. She has not gained any weight since the few months shes been with us. We know its not worms. They have been eating the same dogs as all my other dogs have been eating, purina dog chow. We've never had a problem with this food. All three of my dogs have almost 24 hours access to food, but she is still thin as a rail. She was checked by a vet before we got her from the rescue and nothing was wrong with both of them. Is there something we can add to her diet to make her gain weight? I was thinking peanut er to add more protien to her diet.