Lice or mites- Check around her body for any creepy crawlers.
Being bullied- She could be getting bullied and picked on by another flock member that’s pulling feathers out.
Fungal infection- She could be developing a fungal infection of her skin, causing the feather loss.
Molting- Not all birds molt at the same time depending on age, maturity and diet, but, like how every person is different, so are chickens and some molt later.
Rubbing- She could be rubbing her cheek on something, but I think this is unlikely the cause.