Chick will not grow

I have a little different situation but relating story. I have 4 that are 9 weeks old, 2 Easter Eggers and 2 Brahmas. Now EE's are supposed to be smaller birds and Brahmas are supposed to be great big birds that take awhile to mature and reach full growth. One of my Brahmas is a rooster, he is huge for 9 weeks, literally almost as big as my smallest full size Production Red hen. One of the EE's is very close behind him in size. The female Brahma is tiny, she eats, drinks, is healthy but is the smallest of the 4 and I did notice she was just not a big eater. I have been feeding them the usual chick mash starter feed that is all ground up fine. I also give them a treat of fodder
(sprouted pasture seed in my case) most days. I noticed that she was way more quick to take to the fodder treat and is desperate for it. I finally got sick of them wasting so much feed on the floor of the brooder and the other day switched to the starter crumbles instead of the powdery chick starter. It is a different color, deeper green, has more of an earthy smell than the sweet chick mash. She has been gorging herself for 3 days on the stuff. I think she just didn't care for the chick feed. I didn't weigh her prior but just over 3 days I can no longer feel her bones when I pick her up and she actually feels like a real chicken now and I notice the gap in size between her and the other smaller EE is much less all of a sudden.
I am not saying this is the case with yours, you may have a mix or a bantam that will always be tiny but it may be that she will sprout quick at some point too.

I have all the same breed golden sex links. My lead hen is huge compared to the others, like almost a pound bigger and then my runt is half her size! I had to nurse the runt a bit when I took her home at first. She was slow at the feed store and I'm sure they were going to call her a goner so I grabbed her up and took her. I had to wake her up every hour to drink electrolyte water and eat but she bounced back after a few days and took about 2 weeks to fully recover. So I'm guessing she is just smaller. My breed said 4-6 pounds for size an that's a huge difference on appearance from a 4 pound bird to a 6 pounder!