I have problem in downloading videos from youtube.

jdownloader can capture the all video and audio files of youtube link, and download the mp4 video file in two part one part as video dash file and other part as audio dash file, but after completing download process, it can't merge the two parts to make a complete mp4 video file. I found out that mp4 file is only audio file with less size than expected from downloading.

I don't have the same problem with other streaming websites.

Please, help me as soon as possible.