There are two possible reasons for a track without album art to not be displayed in the Cover Art tab:
1.) TuneUp and iTunes are out of sync. If you've recently added the affected album to iTunes, TuneUp most likely needs to be re-synchronized. Restarting both TuneUp and iTunes should fix this issue.
2.) The tracks are missing entries for Album and Artist. TuneUp can only find the album cover art if both entries are present. If you clean these tracks (drag-and-drop them on the Clean tab), TuneUp will be able to locate matching cover art.