Sometimes importing doesn't import all the text/subtitles of a video

Some videos I upload the whole video uploads but only like 1/2 of the subtitles are there, the rest is missing. And sometimes there are more than one lessons per video like (1), (2), (3) etc but not in this case. This video in particular: La géographie de la Suisse - YouTube (although it does happen with other vids too but it is somewhat rare)

That seems like some issue with the subtitles file attached to that video. How often exactly that happens?