Cello — Not getting warranty replacement for 750ML Flask

I bought cello flask 750ml thru online. It is 6 months old. We used the flask only one time. 2nd time the flask didn't work. It is not maintaining the heat. I contacted the customer care thru email. They asked photos of the flask. I replied with photos. Simply they replied that dent is there they won't give replacement. But there is no dent in the flask. It looks new. Just to reject the warranty simply they said 1 reason and rejected. Cello product is waste.