Sometimes when a new menu is released, some items on the menu may be optional. This just means that the items are hidden by default, since not all store may sell them. This can include locality specials and past items or specials. Optional menu items are opt-in, meaning that they must be set as In-Use before they will be displayed in Invoicing.
To check if the item is an optional menu item. Open Backoffice, and click Lists > Menu > Optional Items.
If the item appears on the right column, it is an optional item and will need to be marked as In Use. Select the item and click the [ < ] button to move it to the In Use column.
You can use Search to filter the list to items you're looking for, or use [<<] to move all items to In Use.
Once done, close and re-open Invoicing on all computers to make sure that the item is now showing.