I'm unsure why it's not working. I'd recommend trying to fetch the group feed using Facebook's own tool to see what's happening:
developers.facebook.com/tools/explorer/
In the top right, click the "Get Token" and select the user_managed_groups permission. Then, enter the following query into the GET URL section:
xxxxx/feed?fields=message,from,updated_time,name,link,caption,description,comments,picture,full_picture
Where xxxx is the group ID.
Let me know if that shows the correct posts or if it's missing it as well. That will help narrow things down.
Thanks,
Alex