[Roy Dennis]

When you say download it generally refers to songs from internet or other computer.
If your using the Index song files to import songs from your computer, after pressing select it should list all songs that have not previously been indexed (Unless you untick the box for Hide Indexed tracks)

1) Is it listing all the tracks in that upper window ?
2) If they show in the upper window does it only lists 1 track in the lower section after pressing the Add button ?
If this is the case then it is to do with your define field order.
Sounds to me like your disc Id and track # are combined.
Make sure the Disc id window is not including the track # else it will only import that track.
3) If that’s not the case are all songs are being passed to the bottom window ?
The select all button does not select all if you are trying to import a track that has previously been imported but you should be able to select each individual track by ticking the individual boxes then import all those in one go.
Sorry if I’m miss understanding your problem.