Hello. I am having a similar problem with my 9 month old son with SWCAH. He has been around 14 lbs. for the longest time. At his 9 month checkup a few days ago, we realized he is losing weight, not growing lenght-wise, and his head circumference is actually SMALLER than before! I was pretty alarmed and so was his pediatrician. He called my sons endocrinologist and the endo said to lower his dosage of cortef and check back in a week. If my son hasnt gained some weight by then, something else will have to be done. My son also has a pretty good appetite. I am hoping that lowering his dosage of medication will help. Also, he seems to be lactose intolerant as well. He is breastfed, but frequently seems "gassy" or colicky around 3-5 AM. His pediatrician suggested that I eliminate milk from my diet to see if that will help.
Hope your neice will soon start growing like a weed, and my baby as well :)