SBI — refund issue

I have bought a watch from ethos boutique at 7200. While doing payment the swiping machine didnt accept the card so i had to swipe which the amount got deducted. After few days i came to know that the 1st fail transaction amount also got deducted. So i ask the merchant for refund which they have done within 3 days but the amount didn't reflect in my account after waiting for 21 days. Now it has been 6 month but still i havent received my money. I ask my home branch but they are saying they didnt recieve. The merchant has provided me an arn no 74164528336000869384919 but the bank is still unable to trace it.
So kindly help me with my problem. Refunding was never use to be this much difficulty.