One of the reason may be :
Inconsistency between sourcing rules and replenishment source type setup
Set the replenishment source type to 'Supplier' in Oracle Inventory product.
1) Item subinventory level
The source type is defined in Item Subinventories window of Item and Subinventories forms.
Navigation:
Inventory: (N) Items : Organization Items, (M) Tools : Item Subinventories
-OR- Inventory: (N) Setup : Organizations : Subinventories, (B) "Item / Subinventory"
Go to "Sourcing" tab.
2) Subinventory level
The source type is defined in Main tab of Subinventories form.
Navigation:
Inventory: (N) Setup : Organizations : Subinventories
See "Sourcing" region at the bottom.
3) Organization item level
The source type is defined in General Planning tab of Item form.
Navigation:
Inventory: (N) Items : Organization Items
See "Source" region.
4) Organization level
The source type is defined in "ATP, Pick, Item-Sourcing" tab of Organization Parameters form. (INVSDOIO)
Navigation:
Inventory: (N) Setup : Organizations : Parameters
See "Item-Sourcing Detail" region
x