To me, there's a difference between skinny and stunted. I tell the difference by feeling around on their bones. I have one cockerel who was having digestive problems, and i didn't know it. He started losing weight, and when i finally realized he was having a problem, i could pretty much feel his entire keel, almost to the ribs. He was skinny and starving.
I have another cockerel who has been small since very young, like a runt. He has a very small frame, but he's not skinny. He fills out his frame just fine.
I don't worry about the runt, but i have read that others have helped runts catch up by giving them extra private protein (eggs or egg yolk) feedings a few times per week. Also, i have one friend who swears by calf manna.
If she's skinny, you can still supplement her feeding with boiled egg yolks and vitamins to help her fatten up. And also, check up on her digestive system often.