Hi - exTA (male) here :
Children are often reluctant to answer questions, and some won't answer to their name when register is called.
Does she 'look blank' because she doesn't understand the question? Is her hearing OK? (she can't answer confidently if she can't hear the question properly!)
Are there other siblings? But if she has been like it from quite a young age, could something have happened that upset 'her world' - a house-move; a new adult partner; loss of a relative, or pet? Things that seem totally trivial to an adult might seem traumatic to a child.
A Reception girl in a school I worked in, would not go into the playground at playtime. A classroom door opened straight onto the playground, but she stood on the threshold, watching the other children running around, which she probably saw as really scary! I felt sorry for her, and although I did not work in her class, I asked her teacher if I could try to help. There were posts holding up a roof, for parents to shelter under at home-time, and I asked the girl, if I helped her get to a post, did she think she could hold on to it, so she was still safe but was in the playground. She nodded, so I led her to the post. Gradually I weaned her off her fears, until she could cope with playtime. What I didn't know at the time was that mother had tried to get her to attend nursery, a year earlier, but the child had got so distressed mother had given up.
Sorry about all that ramble, but I hope it illustrates how children can be afraid for no logical reason.
Many teachers don't have the time to 'nurse' kids through their anxieties, but expect them to 'tough it out'.
This hasn't answered your question as to how you can 'help', but as a starting point if you can discover WHY she is like it, maybe that can lead to 'cure'.
If I can help further, feel free to send me a PM.
Good Luck !