What you are dealing with is a low self-esteem chicken, partly due to her breed disposition and her personal temperament. It is possible to rehabilitate a chicken with this problem, but it requires a bit of work on your part.
What I do is to prepare a safe pen in the run, and I place the timid bird in this enclosure with food and water. She's still in the run with the flock, but she gets a few weeks of "vacation" where she can let her guard down and relax, gain weight and self confidence. When she is returned to the flock, she may be better able to find a comfortable place in the flock.
To maintain normalcy, you will need to take her into the coop each night and then place her in her safe pen each morning. My shy hen very quickly learned her pen was where she should go in the morning so I didn't need to do anything more than to open the gate and close it.