My son was diagnosed failure to thrive at around six months old. He was so tiny for so long and didn't even weigh enough to make it on those weight charts at the doctor's office. We had to give him Pediasure just to keep him from losing weight. He's made it back on the charts now, though he is still competitively small. We've never figured out what the issue was and there is no telling if he'll ever just stop growing again. The Pediasure really helped, though. I keep searching around to see if there is some kind of supplement you could give Nan that might help her grow a little more, but there really isn't much info out there on chicks suffering failure to thrive.