I constantly overpay but I’m still in arrears

Dear csahell.

I have been paying csa for 3 children by two mothers, several years ago I asked csa to put two of the children on so I could pay my way, one went on with no problem but it took a year to add the other I rang every month for a year,finally he was added and to my surprise I had to pay back pay for this mistake by csa,

I was told my arrears would be cleared by 2013.2013 is here and nearly past, one of the ex’s has gone on to maintainance direct with is 50 amonth direct debit and the other 160 deductions of earnings but after a phone call to csa yesterday I was told how I’m over 1000 in arears and now the latter ex will get an extra 60 amonth,so now she gets 220 and the other 50,the problem is that they always say I’m in arears but how when I constant over pay many thanks damion godwin