Many owners have complained about the power button getting faulty on their Essential PH1 correctly. What happens is when the power button is pressed, it does not respond even when the screen has turned on. This happens most of the time when answering a call or when the Essential PH1 has an incoming call.
Another scenario where this issue occurs is after installing a new application on the Essential PH1. What you can do about this is to put your phone into Safe Mode so you can test if the power button is really the issue. It might not be about the power button. It can also be caused by a virus or malware from a new app installed. Going into Safe Mode again is where you can also test if the app is what really causes the power button issue. If it still does not work, perform a Factory Reset on the Essential PH1 after doing the Safe Mode process. You should also check from time to time if you have the updated system version of the Essential PH1.