Update Sql Table stock with adding two join tables doesnt work

Question

I want to update my inventory stock by adding two tables values.

The problem is that if the item doesnt exist in one from two tables it doesnt update this item. Why happen this? I'm using Full join in both two tables

WITH cte AS 
(
    (SELECT  
         InventoryItemID,   
         (COALESCE(SUM(CASE WHEN CustomerID = 0 THEN Quantity ELSE 0 END) -  
                   SUM(CASE WHEN CustomerID > 0 THEN Quantity ELSE 0 END), 0)) AS Total  
     FROM 
         InventoryTransTemp  
     GROUP BY InventoryItemID)
),  cte2 as
(
    (SELECT  
         InventoryItemID, 
         (COALESCE(SUM(CASE WHEN CustomerID = 0 THEN Quantity ELSE 0 END) -  
                SUM(CASE WHEN CustomerID > 0 THEN Quantity ELSE 0 END), 0)) AS Total2  
     FROM 
         InventoryTrans 
     GROUP BY InventoryItemID)
)
UPDATE a 
SET TemporaryStock = b.Total + c.Total2 
FROM InventoryMaster a 
FULL JOIN cte b ON a.InventoryItemID = b.InventoryItemID  
FULL JOIN cte2 c ON a.InventoryItemID = c.InventoryItemID 

My query problem is in point TemporaryStock = b.Total + c.Total2

If one from two tables doesnt have any result it doesnt update my stock in specific item.

Should I add this two tables inside the same cte? Without using Join the second table?

Take a look at this screenshot:

Image