Can't see participants anymore when screen sharing on dual monitors

[ChrisE2]

I have been using Zoom very successfully for online teaching for the past 2 years and am a great fan. I am starting to teach a new course tomorrow. When i did a trial run today, I found that something had changed: When I started screen-sharing from a second camera (a document camera I use to do math on), the participants' videos suddenly disappeared and did not return after I stopped screen-sharing. All my settings and hardware are exactly the same as for the courses i taught before. Can anyone please advise me how to get my participants' videos back? I had no problem with this before, and have not done anything differently.  Any help will be much appreciated, thank you.