video problems

On regular zoom meetings with 8-12 participants I have three with regular video problems. 

 Two participants have a "ghost" image of everyone else's  incoming videos occasionally superimposed over their profile video. It is seen on every participant's screen.  Turning video off and on stops it for a time and then the "ghost" images reappear.

One participant has his video on his screen but it does not show up on anyone else's screen.

Is anyone else seen these problems and better still has anyone suggestions of how to fix them.