Here’s a little story that might help someone.
There were two messages in my Junk folder. They were both good messages. I selected one and told SpamSieve to train as good. It moved to the Inbox and immediately came back into the Junk folder. This kept happening. In desperation, I moved it manually to the Inbox. It came back to the Junk folder.
The Log suggested that SpamSieve heard me just fine, so why was it doing this?
Answer: I had started up another computer which also had Mail and also had SpamSieve (an older version). It was the SpamSieve on that computer that was moving the messages.