She may not have the confidence that she is able to do more responsible tasks.
Make sure she has plenty of praise for what she is doing, and explain why she has done something well, so she can see it is real. I.e. that was a really good job, you are so good at seeing the detail in things.. wow, i would never have thought to do it like that, thats a really clever idea.
Then, maybe think of some slightly more taxing tasks, and ask her to take them on for you. Explain that you want her to do them, as you think she is the best person to do it, because of X,y,z reason. That is would really help you out f she would do it. That you rally believe she will do a great job. If she seems unsure that she can, re-affirm that you believe in her, and that of course, you will be there to support her if she needs any help.
Then, explain the task, check her understanding of what she is going to to do, so that she feels confident she is on the right path, and then let her get on, dont be tempted to keep on her back, checking, as that will infer you dont trust her. But make sure, he knows she can as for help, without it being seen as failure. Then, give plenly of positive feedback..
Good Luck.