Just upgraded today to 2.8.5, but this also happened in several prior versions. Sporadically, trackbacks don't get sent. There's no rhyme or reason to it, though it appears that the problem is usually worse with trackbacks to third party providers like Haloscan. Sometimes, I'll have six trackbacks in a post, three will be sent, the others just sit there in the trackback field.
Also, trackbacks don't ever seem to be sent when they are added in an update rather than with the original post. That is, once the post is published, if I put a trackback URL in the trackback field and update, the trackback almost never is sent
Have I got some option set incorrectly?