Checklist:
Your FB app status is "This app is public and available to all users" https://developers.facebook.com/apps/
Your FB page status is "Page is visible to everyone" in page's Settings > Country Restrictions
Your FB page status is "Page is shown to everyone" in page's Settings > Age Restrictions
Your URL points to the right page on your website
Your webpage has this META tag:
<meta property="fb:app_id" content="YOUR_APP_ID" />
The META tag above has attribute property= instead of standard name=
There are no curly braces {} around your APP ID in the META tag.
You enabled mirroring and selected your page in Moderation Tool https://developers.facebook.com/tools/comments/
There is no extra extra slash / in your url
You posted the exact URL in your FB page, as the page administrator
On top of your post you see an Earth icon; move your mouse over and you see "Public"
When you comment on your webpage, you see this note under the comment area: "Your comment may also appear on XYZ's Facebook Page."
When you comment your FB post, you see this note under the comment area: "This comment will also be posted to www.sample.com."
You can fix errors using the FB Sharing Debugger https://developers.facebook.com/tools/debug/
You can clear FB sharing cache associated with your url https://developers.facebook.com/tools/debug/sharing/batch/
Workarounds:
Append # at the end of your URL
Try your URL without www
Bug Report:
You can submit a bug report to Facebook for Developer page https://developers.facebook.com/bugs
Notes:
Comment mirroring takes effect on new webpages created after the setting has been enabled. The first you time share the URL, current mirroring status is retained for that URL - forever.