Touch is erratic/ non-responsive sometime in iphone

My iPhone 6s touch function is erratic or non-responsive sometimes during uses. When I lock/unlock the screen using power button, it starts working again.

I have tried to restart the iPhone, however, the problem persists.

Is it because software issue or some serious hardware damage? I have purchased the phone in 2015 from Shadyside apple store, Pittsburgh and never encounter any issues so far.