My first thought is that there's no way in hell I would make plans to travel to another city to see a provider unless it's someone I had a history with and was dealing with directly. Second, this is why I don't like prebooking since I hear a great deal of stories like this and have had at least two occasions myself in the last 6 months where prearranged deals got cancelled by the girl.
Most of the time, I set something up, at most, the day before. Of course, this does narrow the available demographic but it seems to work for me. Second, with the exception of Kelly Well's, I avoid agencies like the plague. Don't like the additional cost, don't like the additional rules and the pressure that is put on the girls to be much more mindful of the time.
Bottom line here for me is to remember that it's a buyer's market here, especially if you live in a big market city! Go independent, talk to the girl directly and get a much better feel for their reliability. Still doesn't guarantee anything since, let's face it, there more than a few flakes in this business but it might help reduce that risk of the type of experience you've described!