Hen at bottom of the pecking order not gaining weight

Hi, I have a flock of 8 all around 3-4+ years old, with the oldest being 5 or 6. They are all fine apart from the bottom hen, who is very noticeably skinnier than the others, most of which are quite chubby. Of course, being at the bottom means she doesn't get as much food as the others, however she does still eat a lot of food. She doesn't really get bullied apart from the occasionally pecking when a higher up hen thinks she's in the way, but overall flock relations are good. She is still laying and eating a lot of oyster shell and acting normally otherwise, but her keel bone still feels too sharp, so I was wondering what I could do to help her gain more weight.