YouTube might have placed some sort of restrictions on where some of your videos can play. There might be a copyright issue, but that's something that YouTube will have to assist you with.
Here's a link where you can contact YouTube so they can provide you more assistance: https://support.google.com/youtube/answer/7354993#guidelines (see form at that bottom of the page)
In the Other Details section of their support form, mention that you’re receiving a "Video Unavailable" message only when you embed the video inside of a mobile app. Provide specific links to a video that works and videos that don’t work so they can figure out the difference.