Credit card / Vitality /WLH cash back not received

I purchased a holiday to Zanzibar in Dec 2019 with world leisure Holidays and my Discovery card. The holiday was paid up in July. I have been attempting to resolve the issue wrt my Diamond status cash back for months now with no progress. I keep being told that the purchase was done with my old card which was with FNB and now that cards have been migrated to Discovery they cannot pick up the transactions. I have spent hours on the phone and unfortunately had to submit my card statements and holiday invoices several times and still no solution. How can the bank not honor the cash back. No one seems to know how to resolve the issue.