Trackbacks have to be allowed on the post your doing the trackback against. Check with the post author to see if they allow it. If they do, try adding /trackback/ to the end of the permalink (if you didn’t already and assuming it’s on WordPress). If it still doesn’t work, try leaving a comment in the feedback tab on yor admin panel. You may occasionaly see a moderator here, but the admins are ghosts.
nuub, thanks, trackbacks are allowed, I have used them before for the same blogs. Right now, they only seem to work intermitently from wordpress.com.
I wanted to know if I was the only one seeing these problems, hence the post here (I already send enough feedbacks to the admins as it is without checking if the problem is known or not beforehand… ;-)