Clicking to find my reply

Mac Mail has a lovely facility to find your reply to someone from your inbox. You click the up and left arrow icon by the incomimg mail and you see your reply. This is a joy.

Except some days you don't. Instead you get the original incomig mail. And this happens without any changes of settings. And then, by magic, sometimes you do again.

I've tried quitting and restarting Mail. No difference. Synchonising all mailboxes. No difference. Checking the wind direction. No difference.

All my mail accounts are IMAP. I make no changes at all to my settings.

Thoughts, please?