I am also in the same boat on this one. There are random files that seem to not convert when I make the attempt. An interesting thing is if I move a file that won't convert from, say a random container directly into an Actor-Mixer, it converts. But this is very random.
I've verified I'm using Windows / All languages, have cleared my cache, selected convert all, generated all banks. Nothing seems to convert these files to the Vorbis setting that I have.
I have also tried different Vorbis settings and configurations to no avail.