Even more curious, if I can find the same track in a playlist, it will still play, but it still doesn't show in library view. I have tried reimporting - no new files found. If I select "Locate" when browsing the playlist it always finds the track in the tree view on the left - so it knows where the file is.
Any ideas why it seems to be "hiding" certain tracks in library view?
JimH's idea is a good one. You might also have a filter in the View Scheme that is blocking them out (check Step 4 in the
edit view scheme dialog).
Another possibility is that you somehow accidentally deleted them from the library. MC12's import routine includes the option (selected by default) to "Ignore files previously deleted from library". That means that the files won't re-import even if you scan the folder manually.
Playlists generally point to the actual files on disk, so they might still show the files, even if they aren't in the library.
To disable the "Ignore files" option, you need to go to Tools --> Auto Import Options and then "edit" each of the individual folders you have listed in the folder list and uncheck the appropriate box. This option is saved on a per-folder basis.
EDIT: Looks like Jim mentioned this possibility too... Oops!