Using OEmbed, but doesn't display a preview


I'm trying to set up an elgg site and I'm now in the state of add some additions.

(elgg 3.1.1 + oembed 2.0.1)

I would like to display a preview or the media which is linked in a post, like I knew it from e.g. f-book etc. Therefore "OEmbed" seems to be the tool of choice. But after installing an activating it, I see no difference if I add a link to e.g. a youtube-video.

I activated the web-service api and even then, there was no difference. Elgg shows only the clickable link and no preview or media.

Do I miss something, or do I have to do something additional?

Thanks in advance, Susanne.