Additional team and channel-related features and issues within Microsoft Teams for Education
This is likely because you're looking at a specific tab in Teams (across the top), or app (vertically down the left side). Teams does not let you dock windows outside of the main app, so in order to show you the other screens (or desktop sharing) it resorts to only showing the one person talking in a small window. If you double click on the person talking, it will usually taking you back to the 2x2 screen where you can see most everyone (up to 4). Unfortunately, that 5th person won't be visible until they talk or become the focus.