Source:  https://support.google.com/youtube/answer/6373554?hl=en

If a video doesn't generate automatic captions, it could be due to one or more of the following reasons:

• The captions aren't available yet due to processing complex audio in the video.
• Automatic captions don't support the language in the video.
• The video is too long.
• The video has poor sound quality or YouTube doesn't recognize the speech.
• There is a long period of silence at the beginning of the video.
• There are multiple speakers whose speech overlaps.