This is a signature fault on the 6 plus. There is a trace under the touch IC that becomes broken from the phone flexing. To permanently fix the problem the touch IC needs to be removed and a jumper wire installed, you can than put the touch IC back.
If you wanted to try and get a bit more life out of it you could try and put some packing on top of the IC so there is some pressure on it. You would have to search YouTube on this.