Ann,
Could there be a setting in Yahoo mail that is causing the
problem?
Possibly. Or maybe "doubtful".
There is a "Group by conversation" feature in Yahoo Mail, so that might confuse them if they weren't used to it. But it seems unlikely that they would have changed that setting coincidentally. And the one "conversation" I have in my inbox doesn't seem confusing: the list shows a number for the number of messages, and opening the conversation shows the messages in one tall scroll.
One thing I noticed recently was they invited me to try the "new" inbox. I think they rolled that out some months ago, but I very rarely visit so I just now saw it. So I wouldn't guess that to be the source of a "recent" change either.
You could also double-check their subscriptions to be sure that they aren't using options in the Advanced Preferences expander to limit what messages they receive. But adjusting that would more likely cause them to receive only the *first* message of a topic, not the last. So I doubt that's it. And again there's the "recent change" question.
Shal
Shal Farley