Why am I in arrears when I’ve been paying for so long?

I have paid child maintenance for my son for six years.He now lives with me in a permanent basis.I have put in a claim against his mother.

I ahve been told I owe 16000 olus interest plus arrears. We split up when he was 3 1/2 years old.I did not pay fir three years. But have paid 50 then 40 pounds a week for over six years.

While also have had two more children.But have been informed by the csa because it was nit down as child maintenance on standing orders,I am still liable to pay all money owed!!!.Even though have prove of standing orders oayed to his mum with the reference of adam on it.