Unpaid Cashback Benefits

I had a various assupol policies with Assupol since 2000 and during my tenure as a valuable client with good standing I had a cash back due to me ion which I was accumulating throughout the years however my policies lapsed and the cash back account continued according to their terms and conditions as I was informed that I will be legible to be paid in 2015 February. I did not claim my cash back then and finally when I enquired in September 2019 through their call centre the consultant informed me that my cash back was paid into my Standard Bank of which the same account was closed in 2012 was dormant and no longer existing. The consultant promised to forward me an email as proof of transaction (deposit) but nothing was ever received on my side. I again made a follow up and another consultant promised to investigate and revert back but no response was ever received. Again I sent numerous emails on their info line but nothing was received on my side. I have a reason to believe that the company has deceived me and deviated on their terms to pay me what is due to me. I need an investigation regarding this matter.