There are several ways that you can ensure that you join and consistently log into your instructor's course.
Your instructor should give you your course URL, which contains your course ID. Make sure you register for your course from this URL. Also, after you've gained access, you should log in from the same URL.
Know your course ID
Locate the course and instructor's name on the log in screenYou can find this information by looking on the log in screen, on the left side of the page, below the LaunchPad title.