I thought my Speckled Sussex was failure to thrive. She feathered much later than the rest, & was a bit smaller. I made sure she ate everyday, (I’d sit and watch to make sure she got food), and sometimes I would take her out and feed her egg yolk mixed with chick crumble which she loved. She continued to grow, but stayed smaller. I thought maybe she wouldn’t lay, ever, but she lays a tiny cream colored egg. She seems dainty compared to the rest, but is healthy in every other way.
I don’t know what your chick is, but I wouldn’t give up on her yet. Maybe she will pull through, maybe not. But if she’s happy and well-fed, I think you should wait it out unless she seems like she’s suffering.
One other thought, could she be a bantam? (Just thought of that as I was typing).