I'm assuming you are using the "[youtube] [/youtube]" tags. Youtube urls are HTTP Secure (have an "s" after http)
When you copy the url, it will be "http
s://www.youtube.com/watch?v=...."
Delete the "
s" so it is "http://www.youtube.com/watch?v=...."
It may still not work if you are posting other people's vids; they may have video embedding disabled.
If you are posting your own work, make sure that video embedding is enabled in the settings on youtube.
At least, that's what I found from a little testing. There may be other factors at play that I'm unaware of.