email replies not showing in thread

I have used Mac Mail for many years with no problem.

Recently I noticed that when I receive a reply from an email, it shows up in the lefthand column as usual (where I get the list of all emails and just the first couple of lines of text), but when I click on the relevant email it does not show the reply at all. Normally I would expect to see it above my my recent contribution to the thread.

Has anyone else had this? How can I get to see the email replies.

Thanks

Rob