Are you sure the tracks are actual GPX files that are loaded, and not being displayed, perhaps from a GPS download (or Astro live tracking in progress)? Those wouldn't be selectable until you save them out to a GPX file that is loaded below in the active file list.
One easy way to tell -- go to the active file list and hit "CTRL-A" to grab everything. Then right click and choose "Disable All" -- then see if the tracks are still being displayed. If they are, they must be from other source, so they probably won't be selectable.
Hope that helps.