How can the CSA take so much from my severance package?

I am a nrp my children the oldest being 23 the youngest being 21 I have a csa debt they are collecting 4months ago I was signed off sick from work with back problems I’m awaiting an m r I scan.

The csa did an attachment to earnings order on me and gave me a protected earning rate of £204.00 but have been unable to collect as I’m on sick pay.today I was notified my company had not renewed my contract so played me all monies owed including holiday pay so after 4months of living on £87.00 a week I thought the extra would come in handy as I have 3dependent children living with me including my 5year old twins.

So I checked my wage slip found I had been paid £669.67 gross then looked what my take home was £199 so after tax and insurance the csa had taken £343.27.

Now how can this be fair when challenged I was told even if you have over paid you probably won’t get it back if you owe a debt.