Directory does not show up in main window

Directory box shows correct file as selected but - nothing in main window. This occurs with many different selections and the directory does exist.

I am not sure where you are missing the information.

If you select a directory with the functions from the toolbar or the file menu then no files will be displayed but only directories - this is the way the standard dialogue in Windows works.

If you have loaded the files successfully but you do not see any in the files list, press F3 to disable the filter and check again.

I think my problem relates to file type - for example I see
Marianne Faithfull/Kissing Time (FLAC), but not Marianne Faithfull/Come my way (AIFF). So some directories behave and some do not.

You can determine which files should be shown by mp3tag in Tools>Options>Tags.
Mp3tag does not show the files that are not in the extensions list.
But even if you inlcude an extension then it does not necessarily mean that the file can be tagged. E.g. AIFF format cannot be edited.