How do you deal with flaky girls?

So there's this girl I've been hanging out with who's pretty flaky and I'm not sure how to deal with it. I'd say she flakes out 1/4th of the time, but at least she does the rescheduling, so I don't think it's because she doesn't want to see me. She'll usually do it a day or two before the date, so it's not a last-second thing. The thing is that most of the time, the reasons she gives aren't the most convincing, like "I have a headache" or "I need to study", but the test she's studying for is like 6+ months away.

I don't want to let her get away with this, but by acting aloof and not letting it faze me gives her control (which makes me a beta), but I don't really know how to bring it up without sounding bitter.