The only time this happened to me, after being ignored for nearly a year I just threatenened with small claims court, and they paid with in days. I didn't even send anything official.

What I did do was research into late payment law and found that not only would they have to pay the court fees, but also a standard 8% interest on the debt. I think givin them that info helped encourage them to pay up.

Hope you get what you're owed.