If your judgment and decree identified these fees as additional support, you should be able to have the court reduce the amount he owes you to judgment and categorize that judgment as child support arrearages. Then the county should have no problem collecting for you. If they still won't collect it, you can garnish his paychecks. The first step, however, is to have the amount he owes you reduced to judgment. When you were back in court the last two times, didn't your attorney ask for judgment on these amounts?
You should contact an experienced family law attorney.