Having issues collecting payment for services rend...

Having issues collecting payment for services rendered almost 6 months ago. Very hard to get a response from anyone that is in a position to do something about this. When I did actually get to speak with someone (once) or get the occasional email response, I was given many excuses... "I need an invoice copy." "Checks will print next week." "The check will be signed next Tuesday." Then it changed to "There wasn't a PO# so it's taking longer," when indeed there was a PO# issued and was printed on every invoice they ever received. A/P contact recently quit, I would assume this was due to people like me that call and email non-stop trying to get payment for legitimate services rendered... or maybe because there was no money to pay her?