I would venture to say that she is behind the growth curve, maybe way low on the pecking order and not getting enough protein... or just a late bloomer. it's possible she has an issue that prevents her ovaries from working and producing eggs but I'd give her time, maybe take her out daily if you have time and give her some grower formula or chick mix, something high protein. my guess is that she is expected top out as a large bird but looks slight at this point. check her crop as a precaution to make sure there isn't a problem there, but it doesn't look like it from here. check droppings for worms. parasites can slow things down at such a young age. At some point, even down south, light may become a factor. they literally need light to pierce their skull and stimulate the photo receptors in their pituitary gland, these govern ovulation. sometimes folks put their coops where they are under a tree or the roost is super dark without morning/evening sun, they need about 14 hours of light a day to lay. some breeds are better winter layers than others but sometimes environmental or coop position factors come to play. that's my cheap cheap- two cents on the subject. in any case, I'd give her more time before swapping her out.