What are the reasons why a PO is not considered as SUPPLY in MRP?

Some of the lines in our PO are not considered as supply in MRP.  We have confirmed this because MRP drives/creates a new PO when in fact, there is already a supply quantity sufficient to cover the demand.

I read that the following should be checked to know if a PO is recognized by MRP.

1) Is the Item mrp planned? >>YES
2) Is the PO inventory or expense. (look at po distribution) >>INVENTORY
3) Has the po been approved? >>YES
4) Do you see a record in mtl_supply table for the po? >>YES

What else can we check?  This issue has a big impact on us.  Would greatly app