Nan is a cutie pie. She looks so content with her new little friends. I can see it now... every few months you hatch out a few new friends for Nan the enabler! I would think it has to be an endocrine problem.....like a thyroid problem or something. I would definately encourage a wide variety of foods just in case you can find something that helps. Please keep us updated.
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.