Hi,
there are two ways to add objects to a file system backup specification (have a closer look, they differ in the color used for the top-level checkmarks in the GUI Source page). It's made more obvious on the Backup Object Summary page, so have a look there. Make sure you add these servers as Client System objects, not Filesystem objects. A Client System object always includes all the file systems/volumes that are found to be mounted there the moment the backup runs, with Excludes (below Trees/Filters) as a mechanism to prevent some of them from ever being backed up (if you need that - but the exclude list is empty initially).
To get a Client System backup in the GUI, first check the entire host (top-level checkmark), then expand and proceed to exclude certain volumes/files you are not interested in. If you instead first expand the unchecked top-level for a host and proceed to select individual volumes directly, you will get individual Filesystem backup objects.
HTH,
Andre.