Zoom meeting issue with participants

We are a non-profit, HIPPA compliant agency.  At several zoom meetings we have hosted recently, we have heard from many participants that they attended the meeting and could hear everything going on, but were unable to show their video, chat to us, or unmute.  On our side, we did not even know they attended until we heard from them afterwards, because they did not show up on our participant list, nor could we see or hear them.  This is a problem for confidentiality since we are unsure of who is attending.

What is the cause and how can it be corrected?  Thank you