If the Announcements or other Discussion Forum in your course is not causing an email notification to all students, you may need to check the Subscription status of the activity. If a student is not subscribed to a forum, it will not create an email notification. Subscriptions can be manually selected by students or they can be automatically applied in the settings. 

 - Open the forum activity

 - Open the Subscription and tracking section

 - Ensure the Subscription mode is set to Forced subscription

    - if it is not, select Forced subscription and then save at the bottom of the page

    - If it is, select a different Subscription mode, select save at the bottom (to reset the subscription mode), get back into the settings again, select Forced subscription, and select save at the bottom. 

    - This will reset the subscription mode.