"Shuffle" stops after playing one track

I'm using iTunes 11.1.3 on Mountain Lion. I have a smart playlist with 630 songs, which plays fine if I'm NOT using "Shuffle".

However, when I turn on "Shuffle" to select the next track randomly, the playback stops after finishing the current song.

The checkbox for all songs is "checked".

Any thoughts on how to make this work?

Thanks!