Usually this is because one or more required activities for the course has not been logged as completed in your Plus activity log. The best way to identify which activities are incomplete is to click the My progress bar at the top of the related course page.
This displays a list of the course required activities and details the completion status of each. Click on any item in this list to scroll to it in the course page. Where a progress icon is displayed by this item, clicking on the icon will take you to your My Plus activity log page and show the related entry so you can easily update the related engagement by clicking on the activity update button.