Please note that I have a multi-site setup (main site and then two secondary sites), and there have been some other cases where other people have the same issue: Craft 3: how to show the Yii Debug Toolbar?
However, I have yet to see what exactly is done in order to properly fix this. Someone has noted to simply login into a secondary site in order to see the toolbar. I've logged into both of my secondary sites, as well as my main one, but the toolbar still fails to show up.
I've also tried to clear Control Panel caches to no avail. Help would be much appreciated. Thanks!