She’s likely too self-conscious. A general goal for beginners is to get the individuals so involved with the group that they are pulled out of themselves and engage with the activity without being self-aware. I suggest you find ways to engage with her by…
a. visiting her at home if that’s appropriate. Invite yourself there to join in to some active work such as baking with her and her mother. Find a way to have her give you a guided tour of her domain, from the new kittens to her collections to her favorite books.
b. Invite a group of your students (including her) to stay after school to do something together, such as refreshing the wall/bulletin board display, organizing supplies or books on the shelf–anything that gets the group actively involved with each other and you in engaging but non-threatening ways. You could also take them on a walk or do some other activity they enjoy doing together.
c. Find ways to encourage her, by little informal encouragements, to speak out in class without thinking of it as such a BIG DEAL. Questions and answers should be able to flow in somewhat of a conversational tone rather than the child feeling that her life depends on giving the one correct response to a BIG SCARY QUESTION.
d. Find ways for her to have her voice heard in the classroom other than answering questions. Oral reading, describing a picture the class is looking at, reciting, show and tell… If at all possible, draw her out when you accept some of those first graders’ hand wavings to tell little stories about what popped into their minds when you discuss something.