Check whether the posts not being redirected actually have the correct permalink stored in the DB. There's a bug in which canonicals can't be properly guessed because the permalink structure was CHANGED after the post was created.
Try either creating a new post with your current permalink structure, or editing the post_name DB field for them. (There's a plugin for updating the permalinks in DB)
The question remains.
ASSUMING it is still happening:
How do we FORCE wordpress to REDIRECT to the canonical url?
No just adding a link to it, but REDIRECTING to that page.
I changed the permalinks structure of the site, and added a add_rewrite_rule in functions.php, so now the old
/02/20/2008/postname addresses are accepted and canonicalized as
/blob/postname correctly… and the page is found successfully.
But no matter what I do, the canonical just stays as a link in the old URL'd page, still showing the wrong url in the adress bar.
NO REDIRECT is performed.
Ive worked with another website before, and I ended up (after days of trying tens of different pieces of code) using .htaccess redirects.
I know .htaccess redirects will work, but now that I read everywhere that "It should" be doing it… I'm wondering: Should it? and more exactly "how do I FORCE it?