KMail IMAP not fetching new emails

I'm running KMail 5.15.3 (KDE 20.08.3), Debian stable. It worked fine until a 
few days ago, but it has now stopped fetching new emails from my IMAP account. 
It seems to fetch some when I first log on, but not up to the current time - 
for example, when I logged on today it only fetched emails up to yesterday 
afternoon. From then on, until I shut the system down, it doesn't fetch any 
more. It is configured for 5-minute interval mail checking, and I've also 
manually checked mail in the account.  I can still send and delete IMAP emails 
successfully.

Any ideas on how to get it working again would be welcome.

-- 
David Jarvie.
KDE developer, KAlarm author.