Announcements are sent to students according to their notification preferences set in Canvas, the default setting is to notify students via email immediately. However, notifications in Canvas are user-controlled and allows students can set their preferences to what suits them.
To make sure students see the announcements is to select the option to show recent announcements on the course home page. To enable this, go to Course > Settings > More options, and check the box "show recent announcements on course home page"
