Either the flex for the homebutton on the new screen isn't working - or the cable in the button it self is damaged. If it's the flex in the screen - use other screen, if it's a tear in the homebutton flex, there is a problem. Only apple can replace homebuttons for iPhone 7/8. However, recently there has been “invented” a solution that goes around apple. You can buy a new chargeflex, where there is a new homebutton with it, that'll bypass the hardware-check on startup and make you use the phone. However without touch id and in som cases the lightning port won't work with headsets as well..:)