Give us some context! How did the approach go, what was her level of interest, did anything happen that might have resulted in her getting cold feet?
If everything went well and you felt things were going smoothly, maybe just give her the benefit of the doubt and reschedule. If things were meh during the approach, it's possible she's just not keen. In any case I usually use a simple three strike rule - if she cancels three times it's just not going to happen.
You can usually tell in advance if a girl is really flaky when you talk to her, but in any case once you get her out on the date you can press her on it a bit to gauge whether she's a keeper.