Captivate 9 course not consistently auto-advancing slides with audio/narration

I'm in Captivate 9, and the first course I've published (I've used 8 up until now) I have audio on every slide, and some slides are advancing with the audio, but others, the audio for the next slide starts playing, even though the slide has not advanced.  I set the entire course up to auto advance at the end of each slides' audio, so why would it be inconsistent in moving to the next slide?