Mobikwik — LUM300 not working

Address:

Khammam, Andhra Pradesh, 507003

have purchased a Microsoft Lumia 540 in the moth of August 2015, as per promotional offer I was to get 300 Rupees cashback for 12 months that is 3600 Rupees cashback in total if i add 300 rupees every month into my Mobikwik wallet . This offer is also clearly given in Microsoft official website. This is the link showing that offer in Microsoft official site : https://www.microsoft.com/en-in/mobile/campaign-mobikwik-cashback/ I got the cashback only for three months that is it worked for august. September, October and November. From fifith month that is December when I give the code "LUM300" it says... Coupon code not valid or Coupon balance not sufficient..so please help..My Mail id .. [protected]@gmail.com My Ph number [protected]