I've noticed that the latest 8.0 release seems to have an issue when manually running the DB Tool, Pick up known tags, actors and sites. It does seem to be running during the Add Video Files and Add Video Folders process correctly, but when manually running gives mixed results.
For instance, I have an existing video with the actor 'Matt.Damon' in the file name. The actor wasn't listed at the time I imported it so I manually added 'Matt Damon' and ran the DB Tool but it returned 0 results. I changed it to 'Matt.Damon' and re-ran it with the same results. I then deleted the file and reported it and it automatically added it right away.