I emailed an agency early this week and got a reply that same day (within minutes).  After a few exchanges I have not heard from them and wondered if this is normal.  A few days later I followed up to see if I had booked the lady, day and time but no answer.  Is this is lost cause and should i just move on?

That is why some girls use them.

1. Some agencies let the girls choose their schedule which can change from week to week.
2. Girls that use agencies tend to have other things going on in their lives (another job, school, etc) and strictly do this as a part time or short term way to make some extra cash.

I would not discount this contact totally, bt you can't sit home waiting for them to come through either.  I suggest expanding your circle of friends and at some future time, circle back & try this one again.  
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scheduled date is within a few hours.