If videos in the carousel aren't playing, there might be a few reasons:
Your phone is in the battery save mode
In this mode, the browser restricts playing video
It's still possible to see the full video, if you click on it
Playing mode is different than "Auto-play" or "Sequence"
For example, "Play on hover" will start playing the video if you hover the mouse pointer over the video or tap and hold on a mobile device.