Answer:
The fact that you're getting interviews is a good sign. It means that your letters and resume are working and that you're making a good first impression. There are at least eight reasons you could be failing:

1. You don't look the part.
   Your clothing is out of style. Make sure your clothing, including eyewear and briefcase, is current. Have shirts and blouses professionally cleaned and starched.

2. You lack focus.
   You come across as too much of a generalist, as someone who can "do it all"--jack of all trades and master of none. You haven't defined what you want or where you fit, and companies pick that up as lack of direction. Without sharp focus you appear scattered and may come across as a "loose cannon." To combat this, develop strong preferences and be clear about what you want and what you don't want.

3. You're overselling.
   In an attempt to "make the sale" you're pushing too hard, or coming across as desperate. You may appear too eager or overanxious. Create a high-impact, accomplishment-oriented resume and let it do most of the selling. In general, listen 75% and talk 25% of the time.

4. Your references are shooting you down.
   Who are you using to support your candidacy? Have you asked them what they'll say? Have you prepared guidelines for them? Do they have your resume? Do you brief them before they're called? Be sure to give employers references they can relate to. Engineers like to talk to other engineers, and attorneys prefer other attorneys. Last point: don't overuse your references.

5. You want too much money.
   Don't gauge your present worth on your last salary. The market may have changed; people with your skills could be in oversupply. Do a quick salary survey to determine realistically what you should be earning. Ask what the company plans to pay for the position. Then be flexible. You can lose out by seeming to care more about salary and benefits than about making a big contribution.

6. You appear difficult.
   In multiple interviews companies have time to uncover weaknesses, character flaws, and problem behaviors, such as being arrogant or losing patience. You must appear co-operative, collaborative, and easy to work with.

7. Someone on the team doesn't like you.
   Many companies hire by consensus. That means nearly everyone has to like you. Technical people often feel their track record "speaks for itself," but that's seldom true. In today's team-oriented environment, you need to make a strong effort to be liked by everyone you meet, from entry-level workers to the CEO.

8. You're not the best qualified.
   There may be others who really do fit the job better.