If you are seeing you arrears rising
If you are seeing you arrears rising, regardless of your payments, than it is likely that interest penalties are being added to it.
That interest will run from 3-10% per month, depending on your state. If you are paying $100 towards the arrears, than you base amount you owe must be less that $3000 for you to be maing any kinds of dent in the arrears, if you are in a state that charges 3%. You need to request a print out of your payment and arrears from child support enforcement.
You may have to have to find a way to pay more per month, or get a loan to pay off the arrears. You would pay far less interest on a loan.