Child support

I went to child support last year for my son and I did the application and put all the information I could think of, in it. SSN for my ex.. Everything. But they tried to say they can't locate him but sent a demand letter to the address I provided. Which was his mom's house. They said they couldn't do anything more till I can find where he is.. Isn't it their job to locate him?? What should I do?