Okay... the missing games are ones that have changed status or name since I installed them and but the old version is no longer accessible from the launcher - it was before the "upgrade". They include:
one that has gone into Closed Beta but for which I was in the Open Beta,
three that were free downloads but you now have to pay for,
four that changed directory names though the game's name has remained the same (technically, change of caps to small letters),
one has been had its download removed from itch.io (possibly temporarily),
they don't show up in my library but I have the files and saves in my config directory and can run them from there (which is rather an inconvenience).