Well that's tough, it could be a lot of reasons.
Having been in positions before where I've had to recruit/hire, I'll tell you what made me not call people back:
1. Badly put together resume; meaning not well organised, poor grammar and/or spelling, irrelevant information, too long (don't need your life story). As a rule, your resume should be a max of two pages, and that's only if you are a professional with a few certifications/designations and important positions worth mentioning. Otherwise it should be a page. I had a lady once give me a resume written on a napkin.....but she was crazy.
2. Lack of relevant skills or education.
3. You didn't give your resume or application directly to a manager or person who would give input on whether you should be hired. You should follow up on your application as well, but only once. There's nothing more annoying to a hiring manager than someone who calls five times to find out if you got their application and whether or not they'll be hired.
4. You dropped off your resume dressed poorly, with a lack of enthusiasm or interest. If you're applying for a retail, sales, or food industry position you want to come across as friendly and pleasant, and not dressed like a slob. Professional positions require business attire even when just applying, and a sense of confidence. You have to go in and apply like it's an interview. Go into your interview like you know you've got the job.
That's just the most glaring things I can think of. Anyone else feel free to fill in the spaces.
Live as if you were to die tomorrow. Learn as if you were to live forever. - Mohandas Gandhi