I'll pass on an idea that has worked for me on several occassions.  I have called companies, normally after hours so I am pretty sure to get a voice mail.  I have left a message that I have tried and tried to get paid for invoices that are old and either I have been getting the run-around or been told "check's in the mail".  I tell them I really don't know what to do.  Then I tell them that I guess I will have to look back on the orders I did for them and contact each of the lenders and ask them if they have any ideas on how to collect.

This has worked SEVERAL times, once getting me a check for several thousand from the VP of a large company.

Just an idea.  Works for me.