WordPress comments box is not visible for recent posts

I have a problem with the integration of the Facebook social plugin for WordPress.
I activated the Facebook comments for my posts but this box only appears on just some posts.

For example:

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I cannot understand why I have this strange behaviour. Everything else in the plugin works fine.

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1 comment

  1. Check your WordPress settings to ensure you have comments turned on by default.

    /wp-admin/options-discussion.php

    [ ] Allow people to post comments on new articles
    (These settings may be overridden for individual articles.)