Disappointed - Cash back only 4% instead of 20%

Purchased a couch from @home for R10 000 on the Discovery credit card! I contacted the discovery call centre beforehand and was advised that I qualify for the maximum cash back of 20% due to the status. This is the only reason I purchased on this card and the only purchase made on the credit card. Discovery only gave me a cash back of R400! How is this possible??? DISAPPOINTING!! I made sure that I would qualify for the cash back before buying on the credit card. I have been loyal to discovery and maintained my status over the years. This is the first time I used this credit card with one of the retail partners in the hope that I would receive the R2000 cash back. Is it worth having this credit card?or Is discovery using false advertising? Please attend to this ASAP.