What to do if the company later told me that the code is invalid

Dear Sir, With due respect I want to know the path. The Problem is, I purchased a Branded Company Food product it has promotion Code, to participation in the game customer has to sms the Secret code to the given no. I have sent the code as per instructed. after that, one thank you message received and after few minute another message received that code is invalid. After that I have contacted customer care, many phone calls and emailed all the images of message and packets. After few days customer care told me that code is correct and I am not a winner. due to third party network issue that message you received. So, I want to know Where is my fault and I could not participate in the game and they told me I am not a winner. Therefore show me a legal way. Thanking you