smart recording chapters

 I have just started experimenting with Smart Recordings. My first file which was 24 minutes long had 4 chapters marked. The second file on the same day which was 3 hours long had a message saying a message saying chapters and next steps are not available, but there was highlighting. Why is that? Is there a time limit to how long the meeting/class is?

Also, has anyone discovered some key things that prompt a chapter break or is it solely by time? Ours seemed like every 6 minutes.