Sometimes there's a syncing issue that will keep a student from being enrolled in their courses in Canvas. The quickest workaround for this to get the up and running ASAP is to manually add the student to your class. To do this:

Log in to Canvas and go to the class you want to add the student to.
Click on "People" in the left hand navigation bar
Click on the +People button on the right-hand side of the screen
An "Add People" box will pop up. 
Select "Login ID" instead of Email Address. **Important -- it won't work if you use anything but Login ID
Put the students 25lastfm username in the box (graduation year + first 4 letters of last name + first initial + middle initial
Make sure Role is set to "Student" 
Click Next
Click "Add users"

**Important: This has to be done as the classroom teacher. TCs/Admins don't have the ability to do this from their account unless they're logged in as you.