There are several reasons why this may happen, and I'll tell you about some solutions which worked for me in this situations:
- Rebuilding the search index
- Activating the indexing for Outlook which incidentelly was turned of
- Actication of indexing for datatypes msg, pst, ost
- Repair PST files with scanpst.exe and and compress them with the compression function of Outlook
- Repair your Office software using the function in your control panel / programs