I was in a similar situation owed a couple grand by a company I had used to work for but was doing freelance for now. I started cc:ing in people I knew lower and lower down the hierarchy. Never got a response from anyone. A while later I heard they had declared bankruptcy, so I contacted their lawyer who informed me that only debts from the last 6 months were collectable, and at this point I'd been fighting with them for a year and a half. I'm still grumpy about the whole ordeal.