How can I be in arrears if I pay more than I owe?

I have two boys and I am meant to pay 16% of my gross earnings, as I am with the Child Maintenance Service.

My earnings have never changed and I have a payment schedule that totals I should pay £1200 more in a year than what 16% actually totals.

I have called and emailed through their portal to be told “We’ll get it out in writing to you…” when it still doesnt explain the difference! Also to make matters worse, they say I am in £700 arrears – when I have never missed a payment or been short?!

How can I get answers and this resolved!