You should look for a company that handles international debt recovery. When you are running a business, it is natural that you will encounter clients who have trouble honoring their obligations to you. It does not matter what their reasons are because it does not change the fact that they are unable to pay you as agreed upon. You are now the one in trouble because the money you are expecting is not coming. You are not running a charity organization so you should not be that accepting of the situation. Running after the debtors can be such a hassle because they will say anything or do anything to avoid paying you.
Do not wait too long before getting a company that deals with debt recovery not just in the states but also internationally. You need professionals to settle all your troubled accounts. Do not waste time calling your clients to settle their debts when they clearly do not intend to do so. You will only be stressed out about it. It would be better if you will concentrate on your work and leave the task of collecting payments to people who have the skills, manpower, expertise, and experience for this. You just have to wait for the payment to materialize.