I was having a similar problem (along with an error message when posting in Safari). In my case, it turned out that there was a server problem that was causing the server not to send back the proper response when posting. When this occurred, pings and trackbacks did not go out. (Perhaps someone here who knows how the pinging/tbing process works can explain why this occurs.)
However, I don’t know yet what caused the server problem, though we are still troubleshooting it and might have an answer soon, and I can’t say whether your problem stems from the same source. It probably wouldn’t hurt to keep an eye on this forum, because if I find the answer to my similar situation, I will post it in the above-linked thread.
In the meantime, bookmark this link, because it allows you to send out trackback pings manually.