Fix search issues by rebuilding your Instant Search catalog

f you search for items in Outlook and don’t receive the results that you expect, the indexing of your Microsoft Windows and Microsoft Office system data files (the search catalog) may not be complete.

Symptoms of an incomplete index include the following:

• Partial or no search results are returned.
• Items that you expect to appear based on your search criteria aren’t displayed.

You can rebuild the search catalog, which restarts the indexing of your data files. The search catalog is a file where all of your Outlook and Microsoft Windows items (data files) are indexed.

(source: https://support.microsoft.com/en-us/office/fix-search-issues-by-rebuilding-your-instant-search-catalog-213a2728-0ef4-427a-9bb2-aed329a59b17)