Unless you ask the person who is selecting candidates for interviews, you'll never really know for sure. But here are some things to think about.
If you're not getting interviews, your resume is the first thing to look at. In all likelihood, there is room for improvement. But even if you have an outstanding resume, there are many other reasons why you may not be getting called:
You don't meet the core requirements of the position
Recruiters have a list of qualifications that the successful candidate must (or should) have and these qualifications may or may not be included in the job posting. Applicants will be screened against this list to see if they meet the core requirements. These qualifications may include things like:
If you don't meet these core qualifications, you will probably not be contacted. The only way you would still be considered is if they get very few qualified candidates and decide to soften the requirements.
You meet the core requirements but other candidates have more desirable qualifications
Job searching is a competition - it's you vs. every other person who applied for the position. Therefore it's not just about how qualified you are compared to the job ad, it's how qualified you are compared to your competitors. I regularly hear from people who express frustration because they meet the requirements but don't get contacted. However they don't know how many other people applied and how qualified they are.
Think of it this way. A recruiter has 200 resumes to consider for a district manager vacancy. If you are one of only 10 that meet the core requirements, you will probably get called. However, if you are one of 100 with the desired qualifications, your chances have gone down. If they are not interested in contacting 100 applicants, they will then use other criteria such as more experience or higher education to shrink the list down to a manageable level. You may or may not meet these new criteria.
You are disqualified for other reasons
Recruiting is subjective and because of that there are many reasons why you may have been disqualified from consideration. Here are just a few:
If you're not getting contacted for interviews, the first step is to look at your resume. However, don't stop there - evaluate every aspect of your job search including the types of jobs you're applying for, your methods of job searching (advertised vs. non-advertised jobs), your networking style and communication, your compliance with application instructions, your age transparency, your online conduct, your cover letters, your company/contact research, etc. and adjust where needed.
Keep plugging away and always strive to make the best possible impression, and you'll soon be rewarded.
All the best!