CMS wants child support money I've already paid

What to do when CMS claims you didn't pay your child maintenance? I have standing order, the money is transferred into my ex's account every week. The CMS claims I owe over 10 000 for the past 4 years which is not true, I have bank statements to prove it. I've sent them the bank statements, they lost it and instead sent me a letter demanding payment of the money to my ex. My ex won't back me up against CMS.
Has anyone ever been in a position like this? What are my rights? I keep calling the CMS but they won't give me any informations. They made a mistake somewhere yet it's me who has to prove I am innocent. I have never missed any payment. Even if I did my ex wouldn't wait 4 years to contact the CMS, she would contact them the next day.

We had a family based agreement back then but earlier this year she went thru the CMS to get more from me.
I would appreciate any help in this matter as I sometimes feel like I'm banging my head against a brick wall - no one can tell me anything informative - just pay, pay, pay - like I am only a daddy cash machine.

You CMS assessment can only be from when they were contacted. They won't back date it.

For them to be going back four years there must be an assessment in place from then.

You need to find out the date of your assessment and get a breakdown of the alleged arrears.

They were contacted by my ex 4 years ago and that's when we put the family agreement in place as was advised. They were aware of it. So, yes the assesment is from 4 years ago but I have been paying for the past 4 years they just didn't have a record of it for some reason.

I asked for the breakdown of the arrears but they refused it. Said they can't provide that information and it's up to me to prove I have paid. I'm sending them bank documents again but I am just worried what they might do as they wouln't give me any explanation to what the process is. I wondered if anyone had similar experiences.