CSA not collecting arrears

My son is 24 years old this year, i have complied with the CSA since they started telling them were his father lived etc. Over the years i have been given letters telling me how much i would be getting etc, but never received anything until the arrears were in excess of £20,000. He has been taken to court and i have basically been granted my arrears deducted from anything he sells, and then eventually when i went to an MP a couple of years ago, within weeks a deductions of earnings order was granted, i received maintenance and arrears paid.

My son then left full time education, so i asked for the arrears payment to be increased to £250 a month as he would be about 68 years old before he paid it all back. Last year I asked CSA to check how much arrears were left to pay, so they stopped deducting from his earnings and told me it was all paid, when i enquired about this further it turned out he still had approx £7000 to pay, they kept writing to him but he would not reply, i have told them on several occasions that he has never complied which is why a deductions of earning order was set against him.

I was then told he has changed job and they need to write to him to ask him to start paying arrears, i informed them again, he has never complied and that is why he has had a deductions of earnings set against his wages, the upshot is, twice i have been told it has been sent to a case worker, the first time was last year and i was told i need to wait 12 weeks before anything is done, I then called a couple of weeks ago and was told again it was going to a different case worker and i need to wait another 12 weeks.

They have never chases my sons father for arrears unless i get my MP involved, he just needs to return the letters from them unopened and they don’t do anything. i think they should be liable for paying the remaining £7000 as it is their fault it has never been taken from his earnings, every year i still receive letters telling me i’m to get an excessive amount taken from his wages and it will be paid into my bank account and every year when i call them i am told it is just a computer generated letter and to just ignore it.

I have asked on several occasions over the years to have letters confirming the exact amount owed and and how much i will be paid, i have still nothing in writing even when i was getting regualr payments.

Can you help me get this sorted?