Whom is responsible on the child support?

My ex was ordered to pay child support, and was laid off last October. Although he is still making money working on and sellling airplanes under the table while getting unemployment.He has tried to pay some out of his unemployment check every month, is limited but works for now being he is jobless still 10 months later. He says he does not have to pay or owe back child support, because legally I have not changed the support order? He lives in California, and I live in Colorado. I have never heard of this before and only hear about it when he wants something back from me. Is it my responsibilty to get it changed? Once a court order on child support is made is that order voided when one hasnt made changes? Is back childs upport voided because its not changed by either party?