You need to use keyframes instead of tracking. You can find them here:
For better understanding of how they work, read about them, or watch the video.
Long story short, you need to create keyframe in the first place, where motion starts. After that you need to have keyframe in every place, where is key place of position. If you will just move the mask, ...
Select all the captions in the Captions window; then go to the top right are of the window, right above the first caption -- here, you'll find a grid made up of 9 little squares. Select the bottom middle and all of your subtitles will align in the center of the frame. Hope this helps!