For some reason, the connection between PowerMail 5.5 and SpamSieve seems to have been lost.
I launch PM, collect mail, SpamSieve doesn’t launch and no spam filtering occurs. If I subsequently mark mail as spam (Fly button), SpamSieve then launches.
Please try going through PowerMail’s the Spam Filter Assistant again, to make sure that you have two mail filters for SpamSieve. Also, you could try moving the filters to the top of the list to ensure that no other filters are interfering.
It’s possible that the AppleScript that PowerMail uses to evaluate spam with SpamSieve is damaged. Please try quitting PowerMail and then delete the folder: