Im sorry I have to agree that talking with the A/P clerk solves alot.  I even talked to the Vendor Manager of one company that was a wonderful client.  Seems the people were so busy at the company that the invoices were not getting from the person I sent the work to - to the A/P clerk.  So what I started doing is sending a report of all invoices to the A/P clerk a couple days before payment to assure she had them all - the ones that she had missing I faxed over to her.  I did that 2x a month because that is how they paid and it solved every bit of the problems.  It had nothing to do with them not having the money or wanting to pay.  But I didn't make nasty calls inbetween etc. - I called and tried to find out why I wasn't being paid and how we could be more efficient together in solving the problem.