Sum request of empty fields (despite using Nz)

Hello;

Beginning with Access 2013 (well, Access in general) I’m stuck on a query.
I’m trying to calculate a remaining number of pieces with a formula like: remaining = initial - out + restock.

But for that I’m trying to calculate "out" as the sum of all my pieces that left. I use in my query a calculated field of the type: "QS: Nz(Sum([Sortie_echantillonnage_table]![Quantité]);0)" because there may be no pieces out yet for a stock. It works fine until I want to group by Stock No ... and then the query returns empty fields (so I cannot perform my final calculation).

Here is the SQL code if it helps:

SELECT Mise_en_stock_echantillonnage_Table.ID, Nz(Sum([Sortie_echantillonnage_table]![Quantité]),0) AS QS
FROM (Mise_en_stock_echantillonnage_Table INNER JOIN Sortie_echantillonnage_table ON Mise_en_stock_echantillonnage_Table.ID = Sortie_echantillonnage_table.[N° ID]) INNER JOIN Reintegration_echantillonnage_table ON Mise_en_stock_echantillonnage_Table.ID = Reintegration_echantillonnage_table.[N° ID]
GROUP BY Mise_en_stock_echantillonnage_Table.ID;

I’m not sure I’m clear (it's not easy to explain ...)

Thanks in advance for your help.