The duration of the scene can be freely adjusted only if the background is a solid color or an image.
If the background of a scene is a video, the duration can only be shortened, not extended. This is because adding video content "out of thin air" is not feasible.
Tips: If you can't tell whether the background of the scene is a video, you can click on it in the bottom timeline and check if there is a Speed option in the upper toolbar. If so, it is a video scene. Alternatively, you can slow down its speed to make the scene duration longer.
