Frustratedly cannot change the bank details on my car repayment account

I have tried contacting you multiple times via email, I have called you, I have spoken to one of your consultants over the phone, yet for some reason I cannot change the bank details on my debit order for my vehicle. It's been going on for nearly two months now that I have submitted the change of bank details, emailed and phoned your consultants. Why is it so hard to change the bank details on a vehicle. I am so disappointed with the lack of service and honestly cannot believe that it is this hard to change the bank details of a debit order..