[exim] I'm flummoxed - why would mail get stuck in /var/lib/…

Hi, I'd like pointers, guesses, or just plain speculation on my
problem! So far as I can recall I've changed nothing...<grin>

Anyway, the setup is:
fetchmail 6.2.5-12
exim 4.50-6
courier-imap 3.0.8-4
All are Debian packages, and I use Thunderbird (Windows), Mutt (for
diagnosis), and SquirrelMail

What happens is that fetchmail happily gets the mail into
/var/lib/mail but it doesn't get picked up and put into the individual
mail boxes. It used to. Until 2 days ago.....

I'm unclear about the flow of information here, and how the mail
*used* to happily get from /var/lib/mail/<user> into ~<user>/Maildir.
Any hints, references, etc would be good.

Thanks,

Cliff