Either she's ended up dwarfed or that's a failure to thrive chick. Every now and then I'll get one that just won't grow or feather in normally like all the others - say one or two out of 100 or more a year.
Don't know why it happens, it just is there. I don't have the time or money to feed one that's going to be months behind and may never do well due to internal issues I can't address, so they're culled. That and god forbid it finally does grow out and I end up with eggs or a gene that makes all from that bird, that slow to grow and fail to thrive.
On a flock strength standpoint that bird is not going the right direction.