When I want to add new movies to the database I use the function "Scan Drive for movies". There it starts:
Certainly, the developers are better suited to try and help you sort this, but maybe I can assist in the meanwhile.
While the AMM software is super easy to use, it is not a 'one-click' solution to cataloging your collection. It does require you to interact with it while it does most of the heavy lifting.
"Scan Drive for movies" is a great tool for beginning
a database, or adding a different
hard drive to that same database.
To add new titles that are located on your (previously scanned) hard drive, it would be best to add them to your collection using "Add by Title", or "Add from File", etc. Scanning the entire drive (Again) tells AMM to add the files again.
I do not start movies from inside the database because my files do not remain on my hard drives.
Perhaps another member reading this will have more to offer.