[Mimedefang] md misses some emails

[Tony]

hardware= raq550 running on strongbolt2 (Centos 4.8)
mimedefang version: 2.67-1.el4.rf   from rpmforge
spamassassin version: 3.2.5-1.el4.rf   from rpmforge

My md-filter contains only one task, which is to reject spam (tagged by 
spamassassin) at smtp. Here is the full content of my md-filter:
-------------------
# Detect and load Perl modules
detect_and_load_perl_modules();

sub filter_end {
    my($entity) = @_;
    my($hits, $req, $names, $report) = spam_assassin_check();
    my $stars = ($hits < 15) ? ("*" x int($hits)) : ("*" x 15);

    # Bounce anything hitting the spam threshold
    if ($hits >= 5) {
        action_bounce("No spam wanted here.");
        return;
    }

    # Add spam-score header
    action_change_header("X-Spam-Score", "$hits ($stars) $names");
    }
# DO NOT delete the next line, or Perl will complain.
1;
----------------------

this filter does work because I see entries in my sendmail log showing 
rejection of emails with the message "No spam wanted here.". So I know 
it's working. What baffles me is I also see the occasional spam email 
landing in my inbox clearly tagged as [SPAM]. Why would md 
'occasionally' fail to reject a spam email? I know there's not much to 
go on here but I don't know what info to provide to de-mystify this.