I feel your pain. Leghorns are supposed to be wonderful layers, and to be getting nothing......
What happened to the others? If you had illness, that could sure have set her back production-wise.
She really doesn't look mature enough to lay quite yet. Her comb ins't developed enough and she looks small-ish.
If you're positive on her age, you may just have a slow maturing hen. It happens from time to time. She may pick up as the days lengthen, or she may just not be a good layer. No...