Ex has been paying CSA for years but I’ve not been receiving it!

I recently learnt from my daughter that my ex husband has been paying the csa money for the 2 children he has with me, for years.  My daughter and son are now 26 and 29yrs old. I was on benefits at the time. I had an agreement with their dad that he had them weekends, holidays, etc, and he always bought them things such as uniforms, clothes. I was happy with the arrangement.  However, he has not spoken to me for years and I now realise it is because he thinks I had all of the money he was paying in. I have never received any payment or indeed any letter from the csa.  Where has all this money been paid to?  why was I never offered it?  What can I do?