Check out the latest Director's Survey on the NRMP site. Many programs only offer a certain percentage of interviews before the Dean's Letters (MSPE's) are sent on November 1st.
I applied to 20 programs and got 6 interviews within the first couple of weeks (mostly from the smaller programs to which I applied). The interview train has come to a grinding halt since then but I suspect it will pick up after programs have had time to look over the MSPE.
If you're not getting interviews, check and make sure your application is complete. Also, different programs have different Letter of Recommendation requirements. If they want a letter from the department chair and you haven't supplied that, well, you probably won't get an interview. Check every program's website for their application requirements.