You may not notice it with the naked eye, but it is possible that the fingerprint reader has a layer of grease, dust or any other type of dirt that prevents it from recognizing the fingerprint well. To clean it, a soft, dry cloth should suffice. Of course, avoid being wet and do not apply it on the surface of the reader with great force. Simply rub it several times, in a circular and linear fashion, until you make sure that the sensor is completely clean.