>What typically leads to full images (including thumbnails) being unviewable?
If there are many images and thumbnails missing in the same thread or within general time range, then typically they were never archived for some reason. Image and thumbnail are fetched only after that post has been inserted to the database. Database inserts may be delayed during extreme server load or when the board in question is moving very fast. Meaning that if a post was inserted after being removed from 4chan then that image will never get here. There may be some other cases where image never made it to the archive, but that's most common reason. Of course images can also be removed by archive moderators. Archive mods can also ban images and then that image will never appear in the archive again and will display 'banned-image.png' instead. Then there were the routine data purges that we used to do. In that case thumbnail was retained.>Is there any chance of them ever being recovered?
In some cases, yes. If image was removed during routine data purge then it might be in these collections https://archive.org/search.php?query=4plebs