Butterfly Gandhimathi Appliances — Butterfly gas stove

So we purchased a butterfly gas stove in Jan 2016 and were informed by the seller of the goods that for good atleast one year there will not be a problem with it, as soon as we got it within a few days there the issue with knob to put the gas off and on, the knob just cracks. we first thought it may be a one of occurrence and the seller just gave us different knobs and we changed it, then the same thing happened again in jan 2016 and we complained to them, and they came and changed it, then on 21st may 2016 where we were charged an amount of Rs 20 to change, then again another one on 6th june 2016.. and this has again happened again on the 18th june 2016 which I will complain now. I have the complaint numbers, and the problem here is if the knob issue carries on and there is any leakage we may have a problem with somebody just getting killed, we have tried to tell this everytime to the team there but doesn't seem to be getting a positive response wanted to understand what we can do.. as this becoming a problem and would like to take the matter further as the their team confirms as they get such complaint from many other customers.