Why can participants not see the video on their mobile devices ?

You must first activate the mobile compatibility of webcams on mobile devices. To do this, the host must use the serrated wheel to access the multimedia settings and then in the video options, check the Activate video mode for mobile devices box.
By checking this box, a less performing video codec will be used, but people on mobile devices will be able to see the videos of participants. If the box is not checked, a better video codec will be used but users on mobile devices will not receive the videos of participants, only the voice. They will still have access to the other multimedia functions though (documents, annotations, screen sharing etc.)