Unfortunately, the tracking for character busts only extends up to to when bust tracking was first implemented, so older saves may be missing unlocked entries. So to unlock the entries, you may need to revisit the characters on your current game, or start a new game to visit them for unlocking their entries. Unlocked entries work like achievements, so you don't have to unlock them all on one save--you can have multiple different playthroughs and unlock different entries as you go.
Sorry for the inconvenience as we hadn't had a proper tracking system for bust art until one was formulated during the process of implementing the art gallery (which was about 6 months or so back, if I remember correctly). Note that the entry only unlocks once you see the image (when the game triggers the image for viewing). So to clarify, the character encounter itself does not flag whether or not an image is unlocked. Hope this helps.