...are in high demand, and get booked up a week or more in advance. Trying to make a short notice 1st time visit will not usually work. As you visit more, she'll be more comfortable scheduling shorter notice visits, but initially it will be difficult.

The next time you communicate with her, set a firm date and time. That makes it easier on her to make sure she is available, instead of leaving it to open. She will accomodate your time, if she's not available, she can let you know the next days she has open.

Yes, many popular providers are hard to schedule with, although it can be more an indication of her poor business practices. Two attempts with no results is not unusual.

Unless you're in an area with a surplus of great providers, I wouldn't give up yet. It sounds like she's worth it. Send her one more email with your preferred appointment time, plus other days/times that could work for you. Set it out a few days. Don't load up your message with a lot of explanation or conditions. Be concise.

There's no point in agonizing over whether you blew your chances with her before--she was probably just busy. Don't take it so personally, and don't fixate on this chick so much. Either it works out or it doesn't.

I  know what it is like to find you have the opportunity only a couple of hours ahead and then can not find anyone available that is well reviewed. Like everyone said if she is well reviewed she is probably well booked in advance and it is pure luck to get a reply back in time for a same day appointment. One lady I saw did not get back to me until two days later, but I used that reply to find a good time in the next week to get together. Sometimes you might get lucky but that is luck with a well reviewed provider. Remember if they are  busy with clients they wont get back to you right away. Plus there can be a very busy family life, full time student (and not just the younger providers), plus other delays. The fact that she is responding indicates your not on her bad list. You just need to focus on finding a future time that will work. Give yourself at least three to four days or longer to set it up and you should experience better results.
Unfortunately cancels do come up for legitimate reason as well as bogus ones. If she at least contacted you then she is worth giving another chance to. I would not see someone who cancelled several times in a row, but one maybe two for obviously good reasons is not a big problem. My first ever GFE provider took me almost two months to see, not with cancels but with trying to work out a time. I would call her each week and find we both did not have a time that would work (plus she was not directly in my city) and then talk a little before waiting another week. When we finally got together we were like old friend and it was definitely worth waiting for.