The reason that some photos or posts may not be displayed in your Facebook group or page feeds for some users, is that some users may have the Privacy setting on their personal profiles set not to allow apps to access some or all of their content. By preventing apps from accessing their content then they don’t show up in the Facebook API and the plugin isn’t able to display the content for those users. In order to allow the plugin to display their content they 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 in the group. 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.