Select from two tables

I CAN'T CONECT TWO COLUMNS ITS SIMPLE AN JUST SIMPLE SOLUTION
WHERE I WRONG?

Query result need instead null to be values

select x1.sifra, x2.naziv, (CONVERT(decimal(16, 2), ISNULL(x1.Nabavka, 0))) as 'Nab. (kol.)', SUM(CONVERT(decimal(16, 2), ISNULL(x2.Prodaja, 0))) as 'Prod. (kol.)', 
CONVERT(decimal(16, 2), ISNULL(x1.Nabavka, cast(0 as decimal(3,2))) - ISNULL(x2.Prodaja, cast(0 as decimal(3,2)))) as 'Stanje (kol.)' 
from(select naziv, sifra, sum(kolicina) as Nabavka 
from(
select roba as naziv, sifra, convert(float, kolicina) as kolicina from vp_kalkulacija_roba where id_fakture IN(select id from dbo.vp_kalkulacija_lista where magacin = (select vp_magacin from dbo.podaci_o_korisniku)) 
) a group by naziv, sifra) x1 
FULL join(select naziv, sifra, sum(kolicina) as Prodaja 
from(
select roba as naziv, sifra, convert(float, kolicina) as kolicina from vp_faktura_roba where id_fakture IN(select id from vp_faktura_lista where magacin = (select vp_magacin from dbo.podaci_o_korisniku)) 
) b 
group by naziv, sifra) x2 
on  x1.naziv = x2.naziv 
group by x1.sifra, x2.naziv, x1.Nabavka, x2.Prodaja

Query result

sifra	naziv	Nab. (kol.)	Prod. (kol.)	Stanje (kol.)
NULL	NULL	0.00	1.00	-1.00
NULL	NULL	0.00	50.00	-50.00
72096099	CADRE UNITE DE COUPE	20.00	0.00	20.00
800-40962	SPANNBOLZEN 1 ZSU-400	7.00	0.00	7.00
800-40964	SPANNBOLZEN 3 ZSU-400	3.00	0.00	3.00
800-40971	PRISMENBACKE ZSU-400	14.00	0.00	14.00