Check the target shelf first
Read the progress information shown in your game and inspect the relevant stall. According to the official feature description, shelving automatically selects an appropriate item; you do not need to hold it actively. A different object visible in your hand therefore does not by itself establish that a required item is missing.
Search one area at a time
Retrace the sections you recently visited and view shelves, corners and nearby objects from several accessible angles. Keep track of areas already checked so that you do not repeatedly search the same place. This is a general search method, not a verified map of hidden item spawn points.
Report a specific location
For a persistent problem, record the stall, visible counter, item appearance if known and a screenshot. The official sources cited below do not provide every item location. Avoid unrelated inventory-assistant advice or promised reset commands. See underground-item checks for apparent clipping and completion guidance for progress planning.