I've posted some detailed information in Radio Userland's forums about what I believe is a bug in Radio's implementation of trackback.
The nature of the bug is this: When a post is first published with links that Radio will send a trackback ping to, sometimes those pings are not successful on the first attempt. If the post is re-published, Radio never attempts to re-ping those failed trackback urls again.
Workaround: In weblogData.root, find the failed trackback ping data in
weblogData.posts.[postNumber].trackback.outbound.urls.[failedUrl]. Change the value of the flPinged field from true to false, then re-publish the post to force Radio to re-attempt a trackback ping.