You are probably seeing this because you are facing difficulties adjusting the time durations of your captions/scenes.
Please navigate to Video Settings>Timing to set the same.
Once you are there, You will see something like this:
The section is divided into the number of scenes you have on your editor section. Each has a drop down settings menu.
Here, for example, I have set the timings of Scene 3. The total scene time will always be equal to the end time of the last caption.