Check the publisher logs and see if that provides any clues as to what happened. I've seen a similar bug before and if I remember correctly it had something to do with preloading the document on the server. Meaning, if you select to preload it then the task does not run. Very strange. If the log files don't provide a clue, I would recommend bringing this up to Support (firstname.lastname@example.org). If you do end up opening a support ticket, can you please post the solution here?
We had similar issues, I can't remember for sure which versions. Sometimes tasks just wouldn't run. There didn't seem to be any reason, they just didn't run. That SEEMS to be resolved now, and we're currently on SR4. But I thought it had started working correctly with SR3. Perhaps my memory is off, though.