Sift uses Umbraco's Examine indexes to perform its searches. This provides a fast and efficient mechanism to return the results in a speedy manner. After installing Sift, or in some circumstances after changing the configuration, you will need to rebuild the Examine index.
This can easily be done using the Umbraco back office. Go to the Settings section and select the Examine Management tab. Sometimes, you may not be able to see the tabs, in this case click on Settings in the top-left corner of the page, under Content and above Document Types.
Select the ExternalIndex link and scroll down to the bottom of the page, until you see the Rebuild Index button.
Click on the Rebuild Index button and you will see the following warning. As it explains, the rebuild may take some time, depending on the amount of content that needs to be re-indexed. Click on the OK button and wait for the index to be rebuilt.