Myntra.com — Cashback not received after using HDFC Bank credit card for payment.

This is regarding Myntra order id#1157170 [protected]. At the payment page, it was showing message that you will get instant cashback discount of 10% on using HDFC Bank credit card for payment. I used my HDFC Bank credit card for payment. The page was even showing the discount amount after successfully entering my HDFC Bank credit card details. But on submit, the website showed message of some technical error. I retried the payment in same way using that same HDFC Bank credit card. This time the payment got successful, but I didn't receive any cashback. I tried to get call back from Myntra almost 5 times on this issue. But I haven't received a single call from them. I also tried to inform them by sending a message. But they replied that there is no cashback offer running. What nonsense it is? Why do customer needs to suffer due to some technical error at your end. I was supposed to get 10% cashback for using HDFC Bank credit card. That offer was there at that time while I was purchasing.