Are you seeing a broken-image icon, or is there just nothing displayed where the site icon should be?
You could post a link to your site here, or in a PM that you send to yourself (or me) on Meta to rule out that it’s an issue related to your Discourse forum.
I’m not sure if any of the online oembed debuggers return data about the site icon. Normally for issues related to site’s not creating Discourse oneboxes, I’d suggest trying here: Explore Embed | Embedly, but I’m not sure that will give you information about the icon/favicon.