Hi Dreamy and welcome to the forum! Greytness is spot on, you have to learn what her body language means. I'm also a rehomer, and had several bites before I learned a little more about what Timber was really "saying" with his body language. Also remember that your bird is a prey animal and trust comes very slowly. I had to remind myself that Timber was biting out of fear, not out of meanness. Didn't make it hurt less, but helped me understand. Take it slow and steady and you'll get there. Trust me, though it gets very frustrating, developing a relationship is one of the most rewarding things you will every do (it was for me anyway). It took Timber and I about 6 months to adapt to each other, for him to trust me and for me to respect his fears and body language.
The background thing is important, and something a rehomer often can't know. For example, I quickly learned that Timber was terrified of hands. Why, I don't know, but probably a past experience. He will only step up on my forearm. Also, I learned that the "step-up" command didn't work for him. Again, I'm assuming something in his past. Instead, I say "want to go?" and offer my forearm. If he wants to step up, he will, if not he will pin his feathers a bit (slick them down) and turn his head slightly away from me while side-eyeing me. If I pushed it at that point and insisted, there's a really possibility he'd bite even after 9 years.
Good luck, give it time, and God bless