I've had a LONG run of possible buyers who exhibited every buying signal, knew my rates then dropped out just before the deal closed. Is it something in the air? It's never been like this before.
Be sure to not provide a full proposal of Service Prior to initial payment. Why? Unwittingly detailed proposals often give a prospect the "how-to" implement your service, which they are free to share with a lower cost (and often less experienced) firm or execute your service themselves. A scenario that happened to a 90's tech client when they thought they'd landed a whale.
Instead, provide a summary of service which highlights what you will implement but without the detailed instructions.
Don't let non-starters derail you. In a perfect world prospects will detail why they didn't hire you, but often they won't.