[maildropl] fickle filter sometimes fails when filtering Spam

[darren d. <da...@3x...>]

hey all-

i'm running maildrop 1.8.1 on FreeBSD 5.4-STABLE. I'm using maildrop as 
my LDA with qmail+vpopmail+dspam, and i have the following filter set up 
to enable server side filtering of spam:

   if (/^X-DSPAM-Result: Spam/)
   {
     to "$MAILDIR/.Spam"
   }

This has been working great until recently, when I've started receiving 
some blasts of spam from a single source - 20 or 30 copies of the exact 
same message, but only 30% of them get correctly filtered. I've 
confirmed that dspam is definitely tagging them correctly, but maildrop 
is failing to filter on the header tag. I'm happy to attach the email if 
it helps in tracking it down, but i'm just wondering if anyone has an 
obvious things i should check first?

thanks in advance for the help,
darren david