...I always have a Plan B.

I travel 4 1/2 hours one-way to play in the nearest big city, so I plan as far ahead as I can. You still have some time to hear back from provider A but it's a good idea to have someone else lined up. I wouldn't make it for the same day and time, assuming that you will be in that same area for a bit, but try to have someone else "pre-qualify" you and make arrangements to meet.  

You might go as far as to set-up a meet & greet with provider B to make sure you're both compatible, which could then be a prelude to an appointment.

That way, should provider A fall through, you're trip isn't a waste of play opportunity.

book two with an hour or two in between.  Assuming you have the time.  I'm kind of in the same predicament and am curious to hear everyone's input.

I would do some more research in the area and come up with a few alternatives.  

Then 5-7 days before the day you are trying to schedule I would reach out to the one you have been corresponding with by sending a friendly "hey just checking in wanted to see if [date/time] works for you" email.   If you don't get a response in 48 hours then reach out via email to the top choices on your backup list and ask if they are available for that date/time.  

Hopefully between the various responses, you will have a few options and you can finalize things with whoever seems to you to be the best choice.  I guess ideally for you, the one you have been trying to see would respond during that time frame and before you have booked with someone else, but at the end of the day, as long as you end up with someone you want to see at a price you are comfortable with then you should be in good shape.  I wouldn't consider myself to be committed to the one I initially contacted, nor would I necessarily lower my standards just to book "someone."  

Note that if someone responds and you don't end up booking with her, the polite thing to do is to send her a quick email telling her you booked with someone else so that she can free up that time slot.