Facebook recently changed the way it manages groups. Previously, when you were a member of a group and someone posted to it, you always received a notification. Now, you only receive highlights. No one knows what way Facebook manages these 'highlights,' so you stand a chance of missing something important.
To fix this, you need to go to the group: https://www.facebook.com/groups/wassup.collective/
and at the top, just under the banner, click on the notifications tab. Ensure 'All posts' is selected.