When we first got Arthas from the shelter, he was super terrified and nervous. So, to help him get used to us, I started out by sitting beside his pen and just talking to him. He spent the first couple of months in our bedroom, so at night, my husband and I would lay in bed and talk to him, so that he could get used to our voices.
Next, I would sit in the corner of his pen and just let him come up and sniff and check me out on his own. Sometimes I would give him a treat when he came up to reinforce the fact I wasn’t going to hurt him and good things happen from this human. Eventually, the price of a treat was a pat on the head and we finally got to the point where we could freely pet him. After awhile, he became excited to see us when we came in and did zoomies and binkies when we walked in the room. 🙂
That being said, he is still a very shy and nervous bun and it really doesn’t take much to upset him. So, I think some buns will open up, but may just be hard wired to be extra anxious.
You may also try playing some music or even a TV show in Foster Buns room (not too loud obviously) so she can get accustomed to other sounds and voices and such.