Function doesn't work properly

Hi all.

I wrote a function and it does not work properly. What am I doing wrong?
*- Here is the script .

---
CREATE OR REPLACE FUNCTION "public"."cxp_balfactura" (x_codcia char, x_suplidor char, x_factura char, x_monto numeric, x_moneda integer) RETURNS numeric AS
$body$
DECLARE
cursor1 record;
debitos numeric(14,2);
creditos numeric(14,2);
balance numeric (14,2);
BEGIN
balance := x_monto;

--It doesn't execute this lines, why?

for cursor1 in select cxp_mov_item.tipo_transaccion,sum(item_monto+item_descuento) AS monto
from cxp_mov_item,cxp_mov_head
where cxp_mov_item.cia_codigo = cxp_mov_head.cia_codigo
and cxp_mov_item.mov_codigo = cxp_mov_head.mov_codigo
and cxp_mov_item.fact_codigo = x_factura
and cxp_mov_head.cia_codigo = x_codcia
and cxp_mov_head.supl_codigo = x_suplidor
and cxp_mov_head.id_moneda = x_moneda
and cxp_mov_head.mov_estado <> '*'
GROUP by cxp_mov_item.tipo_transaccion
loop
if cursor1.tipo_transaccion = 'ND' then
debitos := debitos + cursor1.monto;
end if;

if cursor1.tipo_transaccion = 'CK' then
creditos := creditos + cursor1.monto;
end if;

if cursor1.tipo_transaccion = 'NC' then
creditos := creditos + cursor1.monto;
end if;
end loop;

if balance is null then
balance:=0;
end if;

if creditos is null then
creditos := 0;
end if;

if debitos is null then
debitos := 0;
end if;

balance := balance + creditos - debitos;

update cxp_facturas set fact_balance = balance
where cia_codigo = x_codcia
and fact_tipo = substr(x_factura,1,2)
and fact_codigo = substr(x_factura,3)
and supl_codigo = x_suplidor;

return balance;
END;
$body$
LANGUAGE 'plpgsql' VOLATILE CALLED ON NULL INPUT SECURITY INVOKER;
--

Thanks in advance.