Filing a motion and affidavit for citation for contempt of court for a writ of garnishment served and no answer has been filed

I filed a writ of garnishment on 3/26/15 it was served to Social Security Admin. on 4/2/15 by Jefferson County sheriff dept. for a judgment I was awarded through district court for child support arrears against my ex-husband who is on SSDI. SSA has not filed their answers to certain written interrogatories within 21 days of service. My next step is to file a motion for citation for contempt of court, do I file it against SSA as they are the garnishee who was served or my ex-husband? I am doing this all on my own as I can not afford an attorney. Any advice is appreciated.