Please register all sides of your finger while adding your fingerprint. Make sure there is no liquid or other material between the sensor and finger when trying to unlock the device. Wipe the scanner with a clean cloth regularly.
Try pressing firmly and, if that doesn’t work, try pressing very lightly. Restart the device. Try the fingerprint scanner again when your phone turns back on.
Make sure your device is up-to-date. Delete all the fingerprints and register them again.