I'm not sure that understand your list of relationships.
I can see that you have
ItemMaster::ItemNumber = StoreRoom::ItemNumber
but can't tell what you mean by "CostMaster / StoreRoom_ItemMaster
as you don't list a table called StoreRoom_ItemMaster
Also, what do you have selected in Layout Setup...|Show Records From for this layout and what do you have selected in Portal Setup...|Show Records From for this portal?
Storeroom is a table
CostMaster is a table
ItemMaster is a table
Relationships are Storeroom and the ItemMaster related by the Item# (this is the relationship I'm using for the Portal
That is CostMaster table, and the Storeroom table related by the Item# (this is the relationship for may cost and pricing fields in the portal)
So you have
Storeroom::ItemNumber = ItemMaster::ItemNumber
CostMaster::ItemNumber = StoreRoom::ItemNumber
So to repeat:
In layout setup... | Show records from, what is selected?
In Portal Setup.... | Show Records From, what is selected?