Finally found the problem.
MB music library files on PC1 have a directory of C:\users\geoff\music\MP3's\
Playlist files are a mixture of same directory plus \\office\my music\MP3's\, both directories work in playlists.
MB music library files on tablet have a directory of \\notepad\d\MP3's, d being the sd card where the files from PC1 were copied to.
Playlist files are a combination of \\office\my music\MP3's\, which work and show up correctly,
and C:\users\geoff\music\MP3's\, which show up as missing, blue exclamation mark.
MB on PC2, that I sync playlists and music library to from PC1 works fine.
I've got no idea how all that happened, and why the different directories work on PC1 and 2, but not the tablet, over my head. But at least I now know why some playlists files won't play. If that helps anyone come up with a suggestion on how to fix the playlists, much appreciated, otherwise it's the old manual fix, one at a time.