I've been able to see some on a few days notice some, I've had to book weeks in advance.  The more notice the better is my advice.  Always wait 2 or 3 days for a reply.

To get your question answered properly you should state how far in advance you made the calls.  If you call someone a few hours before, there could be many reasons why they did not get back in touch with you.  I will try to answer you question as best possible with the information that you provided.
  Providers that screen prefer that you contact them several days in advance of an appointment so that they have time to get the screening information from you and check it out. If you contact one of them for a same day appointment then you more than likely will not get one. If they listen to your message sometime after the time that you wanted to see them, it is likely that they will not feel and urge to respond.
  Some providers do take same day appointments but require several hour advanved notice.  Again, if your timeframe was shorter than what they need, they may have ignored your call.
  Some providers require only a one hour notice.  If you called one of these then she may have been busy with something else.
  You chances of getting an appointment improves if you can set the appointment a few days in advance.

If not being done already, make sure you are providing good clear communication, including your full name, when/where you would to prefer to meet, when she can/should return a call, if she can leave a message at that number, etc. Basically give enough information, and make it as easy as possible for her to follow up on, so she knows when and how to respond to your request.

Additionally, it may also help you to make your first contact an inquiry only, usually by email works best, asking things such as how much advance notice is needed, what her screening process is, etc. Then, once you have gotten that general correspondence out of the way, it will be easier to schedule an appointment for a specific time/date, since you have established what each of you expects from the other.

hope that helps

When I knew I was heading out of town, I was thinking I would see about 4-6 girls, seeing as how the area is known for great providers.  I started e-mailing about 10-14 days or so ahead of time.  I ended up only seeing two girls, but saw one of them twice, so I had three fun nights.  I was supposed to see another from the same agency as one of them, but that girl was sick during my time up there.  Anyhow, I got replies back within a day or two and things were easy to set-up.  Of course, legality is not much of an issue in Canada and, therefore, the screening process is very light by comparison.

If its get to complicated move in. there is plenty fish in the sea/ If you are getting screned, like you are getting a job with thje State Dept. find somebody who realizes life is to short for all the BS.