No cashback received!

For no reason Discovery Insure stopped paying my cashback in December 2015. After I tried to establish what went wrong the call centre said that they had no fuel spend on record. I forwarded the slips as proof and was ensured that this will be rectified with the next payment in Jan. That came and went and STILL NO PAYMENT. I have jumped through all the hoops to get to 50% cashback and for two months Discovery has avoided paying my cashback amounts!! I've used my Discovery Insure card at BP garages diligently yet no fuel is recorded. Massive inconvenience Discovery!!