Keep at it! Some dogs take forever to get a trick that seems real simple to us!
I don't know how you are teaching her this. Maybe she doesn't like you to hold her paw?
You could try teaching her to paw a target first, like a book or a piece of paper. When she does that, you can take the paper in your hand and ask for a touch. Then fade the paper(by making it smaller and eventually asking to touch your flat hand.
If she is not comfortable with you holding her paw, you have to get her used to you touching and holding her paws first