In the properties of Outlook OST and PST files, make sure that their indexing is allowed (it is worth to do regardless of their indexing being already enabled in Windows Search Service settings).
To solve the problem, you should temporarily exclude Outlook from items that are scanned by the Windows Indexing Service. Enable Outlook Indexing by Windows Search Service If you haven’t made significant configuration changes in Outlook for a long time and indexing is still ongoing, then the problem with the Outlook index is obvious. If you have recently (up to 3-4 hours) performed these operations, you need to wait until indexing is complete.
Outlook search index is rebuilt when you add a new mailbox or Outlook Personal Folder (PST) file, move e-mails between folders, etc. Outlook is currently indexing your items.
