Ex-partner is still claiming, but both children are working

Hi my ex partner is still claiming money from myself both the children are working. Laura is 21 and lives with her father. Michael is 19 next month he has lived with me for the last four years.CSA have said that I under paid for both the children in 2011.

Laura was 18 then and Micheal lived with me.My ex partner was paying but got into arrears but CSA has not asked him to pay back.i was told I could not appeal against this decision .please help