Procmail suddenly puts messages into /var/mail/anne

I suddenly realised that I was missing some mail that should have arrived to 
my Inbox.  Reading through pm.log, it seems that all was well until 6 am 
today.

procmail: [16168] Wed Dec 22 06:00:15 2010
procmail: 
Assigning "LASTFOLDER=/home/anne/Maildir/.INBOX.KDE-Lists.kdepim-devel/new/1292997614.16168_0.server.my.domain"

Then, immediately after that is

procmail: Bypassed locking "/var/mail/anne.lock"
procmail: Assigning "LASTFOLDER=/var/mail/anne"
procmail: Opening "/var/mail/anne"
procmail: Acquiring kernel-lock
procmail: Notified comsat: "anne@2542606:/var/mail/anne"

I seem to have a workaround for now, in that I set up a Local account, and 
suddenly 64 messages have been delivered, but it leaves some big questions.

It appears that these 64 messages span a week, so clearly this is an 
intermittent problem.  What could be causing it?  I've used Dovecot with 
Procmail for several years and not come across this one before, as far as I 
can recall.

Anne