Item names are stored in a list that is saved client side (ie. on your computer). This list (ie. the Item Name Cache) has limited capacity, and as this capacity is met, item names are removed from it to be able to save new ones (new names are added as you see the items in-game, like when you open the Prize List for the AC Scratch).
The Candidate List uses this Cache to present you options based on what you have typed into the search field, but this means that any item name that is purged from the Cache will no longer show up in the Candidate List.
However, even if the item is missing from the Cache, you can still search for the item; it requires you have the exact name of the item (not partial) and then you need to hit "Search by Price" or "Search by Order Put Up for Sale" at the bottom, completing ignoring the Search>Candidate List function. This also has the side effect of adding the item back into the Cache (and by extension, the Candidate List).