Anything other than "Yes" must be considered "No", and you should behave just like she said no. So your response so far has been fine. Don't engage her... if she reaches out to you just ask her out again, if she keeps ignoring your request just tell her to let you know when she is available, and go after other women.
Now why does she do this, when she is clearly interested in you? Who cares. Stop caring about what motivates women and what they want..... not only will you be happier, you will actually start doing better with women.
"She's texting me but she won't go out with me... WHY???"
Answer: "Who gives a fvck."
Stop trying to figure out what emotional snowflakes are thinking, because they don't really know why they do the things they do. Whatever reason she has for not saying yes only means that AT THAT TIME, whatever that reason is stronger than her desire to see you.... that is all you need to know.