The usual reasons that results in a student not being able to join the virtual classroom are:
Wrong browser used
The preferred is Google Chrome, Mozilla Firefox and Opera. Advise the student to launch a different browser instead.
Microphone and/or Camera is not working
Get them to click on the Padlock icon next to the address bar (URL). A menu will pop out, enabling them to allow microphone or camera.
Check if their Camera and Microphone setting was accidentally switched off in the Virtual Classroom:
Cannot hear teacher / other participants
This is usually caused by the default speaker settings on their laptop/computer.
Change the video quality - switch it to low (un-check the box) in order to reduce the load on the participant’s connection.