I see this alot so here it goes.
When embedding a youtube video between [youtube ][/youtube] ensure that the http portion of the link to the video does not have an S at the end. https: = blank white box ..http:youtubelinkhere
[youtube ]http://www.youtube.com/watch?v=sGRw6QcT_pU[/youtube] <--right
[youtube ]https://www.youtube.com/watch?v=sGRw6QcT_pU[/youtube] <-wrong
For whatever reason some people get a secured (https) youtube link and it doesnt fly with embedding.
Also something I've noticed. When on mobile even if you take the s off it doesn't work. To correct this is easy.
If your link starts with https://m.youtube then you change the m to www. That fixes it.