Cash back outstanding since September 2019!

I tried out the Discovery Bank only to be disappointed and so I closed my account. Ever since then (September 2019) it has been a huge struggle to get my cash backs! I have sent the form to change bank details twice, emailed multiple times, called multiple times, I even then changed my details on the website because at the times the option was not available... and after months of repeating myself it is still unresolved! I always get told that the details have been changed and cash backs will pay out, but clearly not if I am still complaining! I’ve even had two consultants personally take it upon themselves to “sort” out the problem. What does that tell you about the service then? Highly frustrated and super annoyed!