The old job search model was to run down to the store, pick up all the local newspapers, bring them home and send out a resume and cover letter for every possible job and then sit around waiting for the phone to ring. This was a lousy model and had the great potential to make a person depressed.
With the rise of the internet, job hunters have been able to take this old model and multiply it a hundred fold. The only trouble is that one hundred times lousy is still lousy!
How could a person send out so many resumes and not hear anything back?
A poor generic cover letter could be a problem? Instead of just sending out the same cover letter to everyone with maybe the salutation changed, I suggest that you do the research on each company and craft your cover letter for each job. It is better to send out one properly directed cover letter than 1000 poorly crafted ones. after doing the research on the company, you may not even want to apply for a job there.
Did I mention that you should have your cover letter and resume proofed by some English language person. There is nothing like misspellings to get your application thrown away!
Maybe you give away to much information in your resume? Let's face it, there is age discrimination out there and if you list a job history of twenty five years or a skill that hasn't been used in the last three decades, your resume is really shouting "Old Man or Woman!" The one exception here is if you are working through a staffing agency then your resume should have everything.
Every resume cam be changed to reflect the needs of the job you are applying for. If the job advertisement has a little different wording for a skill they need then you should change the wording on the resume to agree with the job advertisement. Many resumes are thrown right in the trash if the initial screeners can't find the skills they are looking for in the top one third of your resume!
Wouldn't it be better to be the first person a hiring manager thinks of when a jobs comes open instead of having your resume try to stand out among hundreds of other applicants? In my book "Conquer the Losing Attitude of Job Hunters" I describe an aerobic approach to job hunting that takes you out of the house contacting or actually meeting the people in your field that make the decisions on hiring.