Escape the algorithm that other tubes impose on you
Unavailable videos are marked as watched when importing playlist data. They are counted as watched so you don't have to see them. If you want to see them just go to settings and uncheck "Hide watched videos" :)
Playlists without any unwatched videos will be hidden until new unwatched videos are available. Try
clicking
"Refresh Playlists" in the settings. This might take up to
(20 seconds * the number of playlists you have)
Unfortunetly, YouTube only wants us to request ~100 videos/second so to provide a snappy user experience we limit the playlist history to the most recent 2,000 videos. If YouTube ever provides a bulk API for this metadata--to aid long-tail video discovery--we will definitely raise this limit--but for now it would take 5 hours to scan the full ~180,000 video TEDx channel. Not great. If, however, there is a userbase for it I can write a CLI tool that would save a file with the full history to import into this website.
You can submit a bug report here.