When Outlook’s Instant Search isn’t working correctly, you may need to rebuild the Search Index. You can do this from the Windows Control Panel or from within Outlook.
Rebuild the Search Index from within Outlook
To rebuild the Search Index in Outlook 2010 and newer:
- Open File, Options, Search. (In Outlook 2007 it’s Tools, Options )
- Click Indexing Options button
- Click Advanced button
- Click Rebuild
Control Panel
To rebuild the index from the Control Panel:
- Open the Control panel and type „search“ or „indexing options“ in the Search field.
- Click on Indexing Options.
- After the Indexing Options dialog opens, click Advanced
- Click Rebuild.
Find and Delete Index files to Rebuild the Index
While it’s generally better to use the Rebuild button when you need to rebuild the index, you can delete the search index files from the hard drive to force Windows to rebuild the index.
Delete all files and folders in these directories:
C:\programdata\microsoft\search\data\temp
C:\programdata\microsoft\search\data\applications\windows
After you reboot, Windows will start to index your Outlook files all over again.
More Information
Because rebuilding the index is a slow process that may use a lot of your system resources, you can use Advanced Find (Ctrl+Shift+F) until you have time to let the index rebuild.
If you have contacts folders in different PSTs and don’t want to include them in your search, you can remove them from the search by clicking on the arrow found on All Contacts Items and removing
Source: https://www.slipstick.com/outlook/rebuild-outlooks-instant-search-index/
