There are two reasons students wouldn't be able to see their Blackboard course. The first reason is that the professor for the courses(s) in question have not made their course available yet. Once the course is available then the student will be able to see it on Blackboard.
The second reason could be the student added the course recently and I haven't run any updates since last week Tuesday. This week I will be updating all of the students who have added a course within Blackboard. Let me know if this helps.