Procedure does not run its loop and not from the syntax error

Question

The PROCEDURE below does not present syntax error, but when running LOOP it does not update the records correctly, only a few are updated.

CREATE PROCEDURE `SP_ATU_ITEM_NF` ()
BEGIN
  DECLARE v_nota_antiga INT default 0; 
  DECLARE v_itm INT default 0;
  DECLARE v_MAXIDNOTA INT default (SELECT MAX(idNota) FROM NFItem);
  DECLARE v_Codigo_Antigo VARCHAR(50);
  DECLARE v_Codigo VARCHAR(50);
  DECLARE v_idNota BIGINT default 0;
  DECLARE v_Item BIGINT default 0;
  DECLARE v_done bool default FALSE;
  DECLARE C CURSOR FOR 
  SELECT Codigo, idNota FROM NFItem ORDER BY Codigo,idNota;
  DECLARE CONTINUE HANDLER FOR NOT FOUND SET v_done = TRUE;
  OPEN C;

  LOOP_ATU_ITEM_NF:LOOP
      FETCH C INTO v_Codigo,v_idNota;
    IF v_nota_antiga <> v_idNota AND v_done = FALSE THEN
      SET v_nota_antiga = v_idNota;
      SET v_itm = 1;
      SET v_Codigo_Antigo = v_Codigo;
      FETCH C INTO v_Codigo,v_idNota;
      UPDATE NFItem SET Item = v_itm WHERE idNota = v_nota_antiga 
                                       AND Codigo = v_Codigo_Antigo;
    ELSE IF v_nota_antiga = v_idNota AND v_done = FALSE THEN
      SET v_Codigo_Antigo = v_Codigo;
      SET v_itm = v_itm +1;
      FETCH C INTO v_Codigo,v_idNota;
      UPDATE NFItem SET Item = v_itm WHERE idNota = v_nota_antiga 
                                       AND Codigo = v_Codigo_Antigo;
    ELSE 
      LEAVE LOOP_ATU_ITEM_NF;
    END IF;
        END IF;
  END LOOP LOOP_ATU_ITEM_NF;
  CLOSE C;
END