The first to check is if you have 'joined' the group.
After joining you'll be subscribed to the group, you can manage your group subscription notifications in your settings and preferences
/members-area/support/w/site-faq/27343/i-m-getting-a-lot-of-e-mail-notifications-from-the-site-help
If you want to be notified of what's in a forum, you need to 'turn forum notifications on'
Notifications and subscription actions can always be found in the 'Actions' section of forums and for documents/wikis:
