Call them weekly if necessary in order to find out what is missing. If the school isn't contacting you, it is likely that your file is incomplete. Don't sit shyly at home, afraid that they don't like you, while your chances for graduate school fizzle. Personally make sure that your file is complete. An interview is generally not required by most graduate schools, although initiating an information interview can be a bonus on your side.
Many students are reluctant to request an interview.
It's important that you understand the opportunity you can create for yourself by visiting the schools personally. If you are applying for a fellowship or assistantship you are more likely to be asked to interview. For some programs, the interview is also a time to show that you are well-rounded and aware of current events. Recognize that the importance of pursuing a graduate career exceeds that of going to a particular school. One analogy is that of the athlete who plans only for what he or she will do upon turning pro.