Mail "not junk" button not working

[Whitehill]

Recently, for a few messages, Mail started marking them as junk and leaving them in my inbox. (Remember that for later.) I click the "not junk" button and then they look OK. But the next time a similar message arrives, the same thing happens.

In Mail I have

1. Enable junk mail filtering
2. When junk mail arrives, Move it to the Junk mailbox
3. Exempt: sender in address book, sender in previous recipients, addressed to my full name
4. Trust my ISP's headers

See point 2 and recall what I asked you to remember. As to my rules, I have a couple that will move messages to the junk folder, but none that mark messages as junk - in fact I don't believe you can do that.

So, why are these messages marked as junk and why aren't they moved from inbox to junk folder?