My DH and I are currently seeing this from the other side of the fence so to speak.
He has been paying his ex £300 per month under a voluntary agreement for the past 8 years - he has never missed a payment although she has resolutely refused him access.
In September we got a letter from the CSA saying she had contacted them and asked them to do an assessment and take over the collection of payments. It transpired she felt they would "up" the payments now her daughter is in secondary school.
Every step of the way we have responded within 24 hours of receiving a letter, we have sent pay slips, my husband is also self employed so they have copies of his accounts, we completed and returned a direct debit form to them before the end of September. Still nothing has happened - on four separate occasions they have sent us envelopes with nothing inside !!!
They have decided that as my husband has made a loss for the last two years he will only be assessed to pay £14 per week, much lower than his ex was previously getting. However they have told us today that they cannot get the DD set up before the New Year. They will then backdate payments to September (the date the case was referred) and take however much is due as a lump sum and then they will just take £14 a week thereafter.
Since September my DH has still been paying his ex £300 per month as he didn't want his daughter to go without. Therefore she will get extra for September to hopefully January but we can't be sure.
If this is how they go about dealing with "clients" who comply I can only imagine how long it takes to chase those who don't. The word incompetent is not adequate.
I know that my husband's ex will not be happy when she finds out how much less she'll be getting come January but it is her greed that led to this, we have scrimped and struggled to make sure she got her money but she hasn't appreciated that. My DH is intending to pay the extra into an account for his daughter so she'll get it at 16 so our payments won't actually change but we have found it all so frustrating. The CSA will not allow him to pay more than they assess now that they have been asked to take over the maintenance.
Good luck op - I hope you get your money soon, but it seems to take them an age to do the simplest task in the CSA even when the co-operation is there.