When the client/candidate downloads the mobile app for the first time, they will receive a pop up to "allow video". If they did not click "yes", it will not allow access to the camera in their phone. They will be able to see you and hear audio but you will not be able to see them. If they meant to do this that is ok, however, if they want to be on camera so you can see them, they will need to do one of the following:
Go into their Settings on their phone, Under "Apps", find the mobile app. When clicking on it they will see a toggle (on/off) for camera. They can switch that to "on" and video will work. Same goes for "microphone", if there were audio issues.