Can't export mixed movie from GarageBand
I've often mixed sound and music onto movies with GarageBand in the past, with great success. But today it seems like GarageBand doesn't want to play. I use the Share > Export Song To Disk window, select my settings, and GB goes through its paces... but no mixed movie file ever appears. I've tried sending the file to other destinations, tried changing the export settings, no luck. (Yes, I've turned on the Movie Track.)
BTW, on a separate but related note: when I use Expert Settings, I get an exported file, but it contains an audio mix and one still image of the movie!
Am I doing something wrong, or is there a bug that affects others as well?
MacBook Pro (15-inch Early 2011), OS X Mountain Lion (10.8.2), 2.2 GHz Intel i7, 16GB RAM