It could just be that you don't have a resume that stands out from the rest of them. Remember, some airlines get hundreds of resumes a week. It takes time to sort through the pile.

Based on my personal experiences, most airlines rate their resumes based on a points basis. Have a particular number of points and you get interviewed. The company I used to work for required 10 points for an off the street candidate to be called, 13 points put you at the top off the list. IIRC:

1 point for ATP
1 point for college degree
1 point for advanced degree
1 point for 4 year aviation degree (ERAU, UND, Purdue, etc)
1 point for each 1000 hours TT (up to 3 points total)
1 point for each 200 hours ME (up to 3 points total)
1 point for turbine experience
1 point for part 135 experience
2 points for previous 121 experience
1 point for clean driving record
1 point for cover letter/resume impressiveness
up to 3 discretionary points for special awards and other factors that set your resume out.

It could also just be the formatting of your resume. Working in a recruiting department, you would be amazed with what we saw come in the door. I once got a resume on neon pink paper with a cover letter attached that had a graphic of a dog jumping through a hoop. Where do you think that application ended up? (I'll give you a hint... not in the interview pile, but it did end up tacked up on the wall of fame/shame for a while...)

If you're willing to share your resume, drop me an email/PM and I'd be happy to provide some tips for you. ~J