Hi,
Off the top of my head, the main reason that a transaction would show up in the Cardex for one item but not another would be the line type. Check the order details to see what the line types are for the two differing items. If there are differing line types, it could be due to setup of the item master records with different line types or it could be that two different versions of order entry were used. The different versions used would have different line types in their processing options. To tell if the different line types should interface with the Cardex, you can check the order line type setup screens. Hope that helps.
Sincerely,
Heather Baldwin