It is only able to pull the search queries from unencrypted (HTTP) connections. Now that all the major search engines upgrade the connection to HTTPS, this feature no longer functions as it used to. If you can get a device/browser to submit a search query unencrypted, it should appear in that list.
This switch to "HTTPS everywhere" also affected the Recently Visited Web Sites list until a couple years back when someone upgraded the code to work with HTTPS connections (which is only possible because the domain name is part of the initial, unencrypted handshake—necessary because different websites will have different security certificates, but can be hosted on the same server/IP address, which means the server has to know which site to direct the handshake process to in order to retrieve the correct cert for the applicable domain).
Used this and what rs232 wrote below and added a little editing to to make a clear explanation on the Web Monitor page. Thanks.