Flakes happen when you just aren't that important to her... and when you haven't sparked that emotional trigger, or made a big enough impression on her, she doubts herself and her feelings and starts thinking "my time is very valuable! Why would I want to go out/talk to this guy? I don't even know him and he's just another guy..."
Besides that, she's got 10 other guys texting her and vying for her attention...
So - rather than just give you the answer... Let's think about this... Now that you know why a flake happens, how do you prevent it? (maybe... proactively prevent it? Hint hint) -- How do you make that impression and make yourself stick out in her mind as being someone she'd want to spend time with?
You're a smart guy... you know the answer to this

------ Or you could check the thousand or so threads on this forum which already exist on flakes... They all contain the answer.