Need Help- I need to qualify buyers to save from wasted time.

I am looking for scripts, questions, or any suggestions so that I may better qualify buyers.  I have been working with buyers for over a month and a half, pre-qualified with a lender, and supposedly ready to go.  I successfully negotiated a lovely house that was liked by all parties.  Now that it is time to sign the builders contract, and now everything has stalled.  I find out there is another buyers agent with a buyers rep, (he promised me he was not under contract with anyone else) after assuring me that the money is available, now supposedly it is not (for down payment).

I am scratching my head here.  I thought I had all of my questions answered but it appears I missed something.  I feel like I have been duped and my time has been wasted.  Any advice is greatly appreciated.  What do you ask your buyers to get to the bottom of their situation and motivation?

THANKS!