Flipkart — Serial number mismatch issue

I ordered a pair of shoes from flipkart. The item was delivered to me on 15th june and the next day I applied for return as the item had size/fit issues. However the Wishmaster kept rejecting my return request everyday without informing me. I had to go to their office and then ask them what the issue was. They said it’s a serial number mismatch issue. I have been contacting customer care for 2 weeks regarding this issue. Every time they say this will be resolved but they never solve it. The Wishmaster keeps rejecting the return because of the serial number mismatch issue.