When constructing saved searches, care needs to be taken not to include criteria that does not update the modified date of a record. For example, if you include a condition that filters out all inventory with counts set to zero, you may get unexpected results. This is because inventory count changes do not update the item record. As a result, a diff sync may not pick records up if the inventory count goes from 0 to 1. A manual or forced Full Sync will have to be performed periodically.