None of the titles use anything except letters, numerals, an apostrophe, and spaces. They are not very long, maybe four words maximum. The same with the ones that are working and the ones that are not working. Maybe it's the act of changing the title that makes the difference, but it didn't work in every case.
I did read somewhere in my attempt to find an answer that photos without a file extension will sometimes not show up, but adding the file extension made no difference.
I feel completely stumped, and have spend all day trying to find an answer. I've reinstalled iTunes, and experimented with deleting all photos and importing them again. Nothing seems to work, yet I've been syncing this way for many years without any problems. Very frustrating.
Thanks for your suggestions by the way - I learn something useful each time, even though the solution still evades me!