If she had such a rough past, and heaven knows we can never fully know what's gone on in a horse's life, she may just need more time to 'meet you half way'.
With my last horse, Pride, we had a great partnership, but I didn't really know how fabulous is was until I moved him to the last boarding place he was at, and one day after I had put him back out in the pasture he stayed at the gate while I walked back to the car...I went back to the gate, and spent a few more moments with him, and when I went to leave, again, he stood there for a while, but when I reached the car he finally joined the rest of the herd. He had never done that in 3 years of owning him. He would come up and greet me most of the time, but he had never, ever, stayed at the gate waiting for more affection long after I was done playing with him.
You may just have to wait a while longer for it, and when it comes, you will know. I know it can seem like other people have a better relationship with your horse, but really, how much of a working relationship can someone have if they aren't out there everyday like you are? She may be more curious with them simply because they are there, but she may not work as well for them on a daily basis.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."