What I have just discovered, with the help of tech support for my device, is whenever the playlist is edited, and new songs are added, you must go to the "Edit Playlist" option out beside the playlist to SAVE it. This problem had been frustrating me for months, but from what I can tell it simply comes down to making sure your final list is saved each and every time something is added, or else the new songs will not appear each time the device is powered off and back on. My guess is the 20 you have showing were somehow saved when the original playlist was created, but you didn't resave the playlist itself when you added the additional 80 songs. I've done TEST playlists, and it seems to be the solution, at least in my case with my Android tablet.