Meeting Participant's Video Disappears (green square instead of photo/video)

Sorry, I don’t speak Geekese. In tile view, why does the participant’s video disappear and then reappear, several times in a one-hour meeting (I am the host)? This rarely happened until recently. Now it is every meeting. I use MacBook Pro OS 13.6, Firefox (up to date), and ExpressVPN. Participant uses Samsung Galaxy Note 9 and Android. We have not changed our hardware or software.