Due to some recently increased permissions restrictions it has become difficult to display some content from Facebook groups. In order to show the cover photos for Facebook group albums you would need to use a “User” Access Token from the Facebook account of the person who uploaded the photos to the album. You can find more information about obtaining a Facebook User Access Token here. If you don’t use a User Access Token from the person who uploaded the photos then all you’ll see is the album name and not the album cover photo.
For more information refer to this Stack Overflow thread: http://stackoverflow.com/a/27751849/2509767.