Some progress made.
In my scanned photo, the "Capture Time" was blank. (Inscrutably, that shows as "12:00:00 AM" on the Metadata Panel. I determined the value was blank using Jeffrey Friedl's online Image Metadata Viewer.) I changed it to have a real value of 12:00:01 PM and it very quickly synced.
Now I just need to find all my photos with blank or 12:00:00 AM Capture Time. I see no obvious way to do that. I'll look at John Ellis' "Any Filter".
I'm going to mark this as "Correct Answer". I've applied the fix (Change 12:00:00 AM Capture Time to 12:00:00 PM) to several photos I've had sync problems with and it cleared the jam.
I hope Adobe can make Sync from Classic to CC more tolerant of bad data (missing Capture Time in this case).
I've had this problem for weeks and have seen every other thread regarding this out there. This is the first and only solution that has worked for me.