Some of these bugs are relatively minor but on other occasions, it could prevent Outlook from showing any search results for certain configurations. Unfortunately, there are regularly issues with the Search Index either on the Windows side or the Outlook side.
You can troubleshoot Instant Search in the mentioned order in this article. There are a couple of levels at which Instant Search in Outlook can break. How can I get it to completely index my messages? I let it index overnight but nothing has changed. Instant Search doesn't display any results at all, only displays results before a certain date or always shows that there are items left to index.
