Just to explain:

  • 'Add To Library' -> 'Scan Drive/Folder' - this scans the selected drive/folder for files and then adds them into your Mezzmo library. It does this scan once. It will not add new files that are added to that drive/folder. If you want to add new files that exist in the drive/folder, then you need to run Scan Drive/Folder again and it will pick up the new files for you.
  • 'Add To Library' -> 'Add Folder' - this adds a folder into your Mezzmo library (you will see it in the tree on the left). It also adds the files & sub-folders that exist the folder on disk. It is dynamic. When you browse the folder using Mezzmo app or a DLNA device or maintain your library, then the folder (and it's files) are dynamically updated in your Mezzmo library. So if you have new files or have removed from this folder on disk, then Mezzmo will pick up these changes and show them dynamically in Mezzmo and on your devices.

From your explanation, it seems you have run Scan Drive/Folder once and your initial set of files have been added into Mezzmo library. Later, you added new files on disk into the folder. However, these new files on disk will not be seen in Mezzmo. This is confirmed since you did not see them in Recently Added playlist. So you have two choices - run Scan Drive/Folder again to pick up these new files, or use Add Folder to add the folder into your Mezzmo library. I would suggest doing the latter and then schedule a Maintain Library to run regularly so that your files are added dynamically for you and all your playlists are kept up-to-date.