Sorry I don’t have enough experience to really help, but thought I’d chime in just in case. I wonder if she had some hard times before getting adopted to you? It may just need more time. I wonder if you could change up her set up that would help. Not sure what kind of cage but they are happier typically with as much room as they can have. It sounds like you have them out a lot though, so I don’t know :/ (but having hide houses is great). I feel she would be the same if she was somewhere else, I think maybe experimenting with different things to see if you can make her more comfortable could help? Or try to find some new things or treats she might like? Or maybe she just is not bunny that’s very into people. If the other rabbit acts normal then I really think it’s her personality or trust issues from a previous experience, maybe something she wants that the other rabbit doesn’t, etc… (mine like very different toys, etc)
With my 2, if I start petting the friendlier one the other one sometimes comes for pets too. Not sure if that would work with yours to get some brushing done. Also you could put her up on a table and try and groom her, I think most commonly they’ll behave a bit in that situation. I find pet rabbits to have quite complex personalities, and with mine it has really taken a while to figure out their language and how to act around them. They both have different triggers for what will scare them.