Ex is now deceased but still can’t get money back from CSA

I have been paying the csa for many years now direct from earnings having never missed a payment.

I called the CSA late last year to inform them that my 17 year old daughter was no longer in education and had been employed for a year and also she no longer lived with her mother who was recieving the money.

The CSA investigsted ( checked to see if the mother was claiming child benefit, which she was), investigation complete, I still pay.

The CSA had been in touch recently to tell me that the case closed in November 14 and that I had overpaid and would be reimbursed. I was even asked if I wanted the money back.

Today i got a call to be told that I was not entitled to any money back as my noy 18 year old was still classed as a child as the mother claimed child benefit for her. The mother is now deceased and cannot answer the questions.
Result, no refund. Is this correct?