I had problems with a company in Canada (I am in the UK) who owed me a large sum of money. They played me around for about six months until I got fed up with the hassle of chasing them. Actually this was after one of their people had told me that he was too busy to deal with this as he had other priorities.
I sent them a final invoice the next Monday marked from "accounts" (as opposed to being from me personally). I explained that we had exhausted our "chase up" procedures and told them that if the money was not paid by Friday of that week we would be passing to our "Credit Collection Agents". We received full payment on the Thursday.
As you will no doubt be aware there are credit collection agencies who charge around 15% of any money they recover. I would have happily used one of them had it became necessary.