If the phone is an iPhone, it is well established that dead fingers will not work. The technology requires living fingers. It does not actually work with the fingerprint, it senses the fully active filled with blood ridges and valleys in the fat pads below the surface of the fingers' skin. Without blood flow, these tissues are completely different than they would be if the tissues are dead, without blood flowing through the fingers.
Apple designed this so that thieves could not just cut fingers of victims to activate iOS devices to get around protections to activate them, change passwords and thus make them salable.
Thanks for the info and correction. Never knew that.