ItemSearchBasic works for one item but not another?

The following code snippet works for one item but not another. The difference betweeen the two items is that one is a "Non-inventory Item for Resale" and the other is a "Serialized Inventory Item".

However, the only criteria I am specifying for the search is the item's internal id. The search works for the "Non-inventory Item for Resale" but not for the "Serialized Inventory Item".

Why is this? What do I need to change to get the itemSearch to work for the "Serialized Inventory Item"?

[SIZE=2]
RecordRef[] rr =

new RecordRef[1];
rr[0] = new RecordRef();
rr[0].internalId = itemInternalId;


ItemSearch itemSearch = new ItemSearch();