Touch problem in 6

i have been using iphone 6 + since last year. in the mid of december i faced problem while using it i.e. touch screen was not working properly. I had submitted the phone to the Authorised Apple Service Centre locally located in Kolkata, West Bengal, India. They didint gave me any service report because as per them they had ran a diagnosis by which they didnt get any fault so the gave me back the device again last week i faced the same problem again this time they kept the phone for two days and after that they gave me back asking to use the device for some days but still i am facing the same problem so kindly guide me what to do now..

Mayank