Couple reasons for that.
1. Usually the main reason is that there are moderators in the rooms that determine who gets to share their audio and video. This is done to prevent people from interrupting the presenter during their presentation. When Q&A’s are going on for the breakout session, that’s when the moderators will allow people to join in and share their audio and video.
2. If you are a speaker, moderator, exhibitor, or and attendee that was given permission to share your audio and video and can't get it to work, make sure you perform the following steps: