I've done some experimenting. Sometimes IPB wraps a no-embed code around URL's - not sure why.
If there is an extra space at the beginning or end of the URL it won't work properly. Make sure you just have the url.
Paste the URL, then hit enter to go to the next line. It might take a few seconds, but them embed should show in the editor.
If it doesn't work the first time, repeat the process a few times. Usually one will work. Non-embedable videos will not embed.
I believe this problem is being worked on