Video not working on HP Chromebook

We have two brand new HP Chromebooks that our sons started using for online school yesterday. Using the Zoom for Chrome - PWA app, upon joining their class/meeting, their video screen is black (with their display name in white). The “Start Video” icon/button in the lower left of the screen is unclickable. If we click the three-dot menu button on the thumbnail of their video at the top of the screen, we can click to start video - but that only takes away their white display name and the screen is completely black. 

This is occurring about 2/3 of the time they join a meeting. We have also tried using Zoom in the browser and the regular Zoom app (not for Chromebook). The problem is inconsistent and seems to come and go, no matter what we do (uninstalling/reinstalling apps, switching between browser and app, restarting the device, etc). Please help!!