Some video convertors like, Black Magic, VIDBOX, and Thunderbolt have experienced issues with the conference room not finding their camera or a black screen appearing. Simply go here: https://apprtc.appspot.com and join a room. Leave that particular application running while you also have your BigMarker room open. This should allow you to share the correct camera on BigMarker.
These convertors and applications are sometimes necessary for cameras that are not webcams.