Invalid vouchers

I was so excited when I saw MTN decided to up their game and offer more rewards to their customers in the form of various vouchers that can be bought with YellowBucks. I redeemed majority of my YellowBucks for Steers vouchers, only to be told by 3 different Steers outlets that they are unaware of the Mtn vouchers and that it's invalid voucher codes. The whole episode left me feeling embarrassed. How can MTN offer vouchers that are invalid and why is it that Steers knows nothing about the vouchers offered?