Incorrect Bank Account Debited

I advised Vitality Premium to change my banking details for my Planet Fitness National Membership, however they have debited the old bank account for the third time. I initially contacted discovery telephonically and was sent a form to change my banking details, this form was filled and submitted. A received a reply stating that i should submit a 3 month bank statement which was sent. However the banking details have not been updated and the old bank account is being debited which means iam incurring additional charges for returned debit orders. I would like discovery to resolve this as soon as possible.