She might have fought an illness, that was not visible to you. Or she might have had a difficult hatch, or a bad incubation period. So many things could affect her ability to thrive, including the health & condition of her mother hen, the hen that laid the egg from which your girl hatched.
Give her a little extra high-quality protein each day. Along with vitamins as mentioned. I had one chick that grew very slowly, even though she was the largest baby chick, she soon was outsized by the others. When I realized what was happening, I hand-fed her nearly every day for a couple months, to make sure she was getting high quality nutrition for growth. Most days she got a grated hard-cooked egg, or a small amount of steamed fish scraps, or some pieces of high-quality dry cat food or a handful of sunflower seeds. She's doing okay now, and she lays 2 eggs, every 3 days. Still she's the lowest on the pecking order and pretty much afraid of most of the flock, except for her one buddy. I've long since stopped her individual feedings, but every now & then I'll sneak her a special protein treat, just in case she's having trouble getting her fair share.