Sometimes i use the 1&1 hosting plans to install websites.
With shared hosting plan, 1&1 don't allow external urls by default.
Some tutorials speak about putting the parameter allow_url_fopen in
a php.ini file to solve the problem but that was not solving my problem.
So, i wrote this little snippet to force oEmbed to be displayed.
Add this lines to your functions.php file:
// Enable oEmbed on a specific element
add_filter( 'the_content', array( $wp_embed, 'run_shortcode' ), 8 );
add_filter( 'the_content', array( $wp_embed, 'autoembed'), 8 );
I choose to apply it on the_content, and it works for me.
Maybe, it could help you if you encounter the problem too.