Take her to another doctor. People tend to forget that some doctors graduated nearer the BOTTOM of their class than the top. If a baby that young is not able to digest food, there is something wrong.
I would have the doctor do a fecal fat test. If that is abnormal, you may want to have a sweat test and a genetic screen to rule out cystic fibrosis. Another thing to check for is pancreatitis. My son has been having digestion issues and weight loss. We are going through all the test to find out exactly what is going on.
He should try barium and an x-ray. There can be blockages in the intestines that cause symptoms similar to what you are describing.