This is usually a problem at the beginning of the semester as many students register for too many classes initially trying to make up their mind, but then they drop them when things start stabilizing. So, you will see most classes opening up in the late registration period. It is possible that some very popular classes do not open up. But again we are planning to offer some of these classes every semester. So, you can take this class in the next semester. If you are really interested in a class and that class is closed, good idea to show up in the first lecture anyway and speak with the professor. Sometimes professors can sign in students at their discretion.
Please also note that we have recently seen very high demands for certain classes taught by certain professors. The department cannot guarantee that a student will be able to take specific classes in specific semesters. The department can only ensure that a student can graduate with an M.S. degree within three regular semesters (fall/spring) by satisfying all graduation requirements. If this is your third semester and a class you need to graduate is full and no alternative classes are available, please speak with the graduate secretary.