Why did the CSA never ask me for money?

I split with my partner back in 1995 when our child was 6 months old. I informally gave her weekly money for a year or so and then lost all contact with child and mother which continues to this day.

At some point in the late 90s the CSA contacted me and I assume this was through my ex. At the time I was just starting a Uni degree and they asked for no money. I was a long time doing the degree with no real work in between.

Eventually started my own business which I still run now. Always earned decent money, always above board, always paid taxes, had permanent addresses for long periods.

Throughout all these years I’ve received 1 letter every 6 months from the CSA stating I paid £0.00 in the previous 6 months. Not once have I had a request for payment.

My child is 20 this year and I believe all possibility of being liable for payment ends then. What I’d like to know is why the CSA never asked me for a penny when I never hid myself away and have always been in the system?

My theory is that the period of Uni and perhaps early days of self-employment simply threw the system somehow. I also assume my ex never chased it up for whatever reason.