Shy are you posting on the ADD forum?
When you say "failure to thrive," I assume she is underweight and perhaps a bit sickly. Has she been check for food allergies or intolerances? Does she have a digestive problem. I would like into physical causes before going the other route, out of fear that a physical problem may be overlooked. And that could be very serious. If she checks out physically then look to other solutions.
There was a big fuss over me when I was a young child. I would refuse to drink milk. I could not leave the table until I drank it. So I sat there until it turned sour. I never drank it. Some people, particularly Latins, cannot tolerate milk. Is there something similar going on with your child? You might try eliminating food groups, one group at a time.