I think she has commitment phobia. Why do I think so? Because usually, things gradually improve up to a certain point, and right when I start thinking "we're almost in a relationship now", she stops responding, and she writes me things like: "Oh, I didn't check my phone all week-end, didn't notice you tried to call me 3 times."
A few days ago, though, she replied really quickly when I hadn't messaged her all week.