User Guide Update
Answer: For most Auction Catalogs, the default conditions for the Item Catalog will be to display "Biddable" items only based on the Condition set for the catalog in the Select Items window.
In this common scenario, if an item is not considered "Biddable" (What Are Biddable Items?) then it will not be displayed in an Item Catalog configured to display "Biddable" items only.
The solution in this case is to either update the default Items with types condition to include the types you want added; or, to change the item in question to a "Biddable" item type (and also ensuring its other properties are correctly set).
see Configure Items Displayed Items With Types for more information on adjusting the types for the existing auction catalog condition; or,
see Edit Item Details Item | Type for specifics on changing the item Type,
see Item Types for more information on how Item Types work.
There can also be other reasons why one or more items might not be displayed as expected in an auction catalog.
In most cases, if the above is not causing the issue, CLICK to read more to help get things sorted out.