What am I to do when the CSA is so corrupt?

I have now been recieving “csa” for a couple or so years. As my eldest daughters father did not pay csa I soon shut the case down.

But for my 2 littlest children I still recieve it for them which sums up to around £40 per week,but this isnt whats been happening,it seems like the agency which is supposed to be currently helping me are keeping my money for interest as it seems.

As i need the little bit of money that comes in weekly i havent been still getting the correct amount when i have been told by yet another company i should be on £67 pound or so a week?

What am i supposed to do when i am infact surrounded by corruption?