Outlook for Windows
If your search function isn't finding any of your messages or only recent messages in the last two weeks, you may need to rebuild the search index. You may also receive a message "Instant search encountered a problem while trying to display search results. Modifying your query may resolve this problem" or "Outlook cannot perform your search."
- Choose File | Options | Search
- In the "Change the Outlook stores indexed by Windows Search", click Indexing Options
- Select Advanced.
- In the Troubleshooting section, select Rebuild.
- Select Close when finished.
- Click OK to close out of options.
This Microsoft support article has more detailed information on search issues and troubleshooting tips - https://support.microsoft.com/en-us/help/2769651/outlook-search-returns-no-matches-found
Outlook for Mac
In Microsoft Outlook 2016 for Mac or Office for Mac 2011, you receive a "No Results" message when you try to search for an email message or apply a filter to a folder, and task items are not displayed in the Tasks folder. Additionally, when you search for mail items by using the Mac OS native Spotlight Search, your search is unsuccessful.
This Microsoft support article has more detailed information Outlook for Mac search returns "No Results," and task items are not displayed