Connect v12 audio issue: some attendees can not hear all speakers

We are running a week-long new employee training and are finding that 2 or 3 attendees can only hear a subset of the other participants at different times. They can usually hear hosts without issue but when other attendees come off mute to ask questions, they do not hear them.  Any ideas how to troubleshoot this? I can't find a pattern of being on VPN or location, etc.  One was on browser, one was on app.