Diminishing effectiveness of Antispam

It's been almost a week since Antispam has caught anything, and during this time it's failed to catch several spam messages, including some almost identical (in subject line or sender) to those I've flagged as spam.  The Antispam program seems to be loading as usualit's just not doing its job.  Has anyone else noticed this?

 

I'm running Outlook Express and XP, by the way.