Bill of Material Substitutes

If a component on a Bill of Material has a substitute item will MRP look take into consideration there is inventory on hand of the substitute item before it generates a message for the primary item?

Example.

Component A   On hand   = 0

    Substitute item for component A  =   Component B      5000 on hand

Demand for Component A    = 1000

-----   In this scenario we are getting MRP messages for Component A to buy 1000pcs

Is there something in the set up that we're misssing that would not generate the messages for component  A?   Even when looking at the part availability screen the shortages show even if there is plenty of inventory on the substitute item.