with fed. gov-t clients - get on horn with them, they have to close the books somehow... especially if they did not pay since 1991... you need to be persisitent - we have contracts too, our supplier in NJ, and they get "paid" when the ordered item is "received" in the inventory software... a lot of times item is shipped overseas, the person who ordered the item has been out of country, the item physically is received but nobody is there to "close" the order as the person is now across the globe in another part of the world, the contractor is not paid... once they got too many of those on their books, they just talked to the contract rep, and the issue was resolved - the deals were forcefully "closed" from DC and the contractor got paid...

you need to do so the same with your USG clients...