If some posts by visitors to your page aren’t showing up in the feed on your website then this is likely because those users have the Privacy setting on their personal profiles set not to allow apps to display some or all of their content. By preventing apps from accessing their content then it doesn’t show up in the Facebook API and the plugin isn’t able to display the content/posts for those users. In order to allow the plugin to display their content the user would have to do the following:

1) Log into their Facebook profile account and go to their Settings page
2) Click on Apps in the menu on the left hand side
3) In the section which says Apps others use click the Edit button (see first screenshot below)
4) Check the box which says Posts on my timeline (see second screenshot below)
5) Go to the plugin’s Settings page in WordPress and click Save Changes to force the plugin to check Facebook for new posts

This should then allow the plugin to display the content that they post to your page. If a user has decided to block apps from accessing their content then there’s not much you can do to display them without having them change their privacy settings to allow it.