This is a problem with my birds as well. I don't know if you can completely stop them from being flighty, but you can help them trust you more. Sit down so they aren't threatened, bring treats, and instead of reaching out for them, let them come up to you. Don't try to pet them until they are trusting you enough to stand near you without being afraid. None of my adult chickens enjoy being touched, picked-up, and many don't even like being too near me, but during treat time, they will all crowd around like adoring fans.
Is she nearing egg laying age? Some birds get more flighty at this time.