After recently upgrading to 2.5 SpamSieve is no longer catching any spam. SpamSieve is definitely running (I checked Activity Monitor) but literally since the moment I upgraded no spam is getting caught anymore.
Any ideas?
Which mail program are you using, and how did you upgrade to SpamSieve 2.5? Usually this happens if you don’t replace the old SpamSieve with the new one (and leave the old one elsewhere on the disk, or in the trash), or if you fail to double-click the new version to launch it.
It would be nice to know that wouldn’t it? 
I’m using Mail.app (Version 2.1) on Tiger. I upgraded by dragging the app from the disk image into my app folder, replacing the old one.
That should have worked.
Well, please check this page to see if there’s a problem with the rule in Mail. Do the training commands work?
I had a similar problem. After quitting Mail (which also quits SS), I replaced the SS app with the new one. Then, I re-launched Mail which also launched SS. This was my mistake. Apparently, I should have launced the new SS first.
I fixed the problem by quitting Mail and re-launching it (again). Now it’s working.
Yes, unfortunately, that really does matter. SpamSieve 2.5 includes an auto-updater, though, so these kinds of problems should soon be a thing of the past.