How do I get the CSA to take this seriously

The CSA have just come back following my request for a variation and still wont increase the amount my ex has to pay.
She earns a minimum £15,000 a year tax free so the same as about £20,000 per year.
They say that as she is earning less than £100 per week they can't change it.
Why don't they ask why somebody declaring only £93 per week can have 2 mobile phones, eat, clothe herself, pay rent, buy a 2 year old car, insure & tax it, put fuel in, reduce her debt by £3000 per year.
This is a joke. Given the economic situation why aren't the Government out to nail these fiddlers.

if she's earning tax free, I'm assuming you're saying she doesn't declare her earnings? In which case, you can report her online to the HMRC for tax evasion. You'll never know the outcome, of course, but you can do it.

I tried to get HMRC interested and they couldn't really give a damn. Without hard evidense they weren't intesested.
What annoys me to finalise last years tax credits claim I had to send HMRC all bank statements for 18 months, P60, P45s from the DWP whan I was out of work, child care contracts, evidense (receipts) of paying for the child care, building society pass book, evidense of child benefit payments.
Instead of hammering tax paying parents or legitimate claimants why don't they go after those fiddlers in the bkack economy. Oh that will because it is just too difficult.
Is there not a life style examination by the CSA?

Will the CSA not do a criminal compliance investigation? This is where the lifestyle is not consistent with what she is claiming to be earning.
They've been investigating my ex for over 2 years now as he's a company director of 2 companies who lives like a premiership footballer with his Mrs and their kid yet claims not to be employed (but surprisingly doesnt claim benefits either) and it's flipping amamzing that someone without a job can buy a 6 bedroomed house, have a big 4x4 with personalised numberplate and take several foreign holidays a year!!
CSA arnt good at communication.
If I were you I'd ask them to do a criminal compliance on the case...but dont expect quick results!
Lou xx