Some agencies have better reputation than others. Do not contact them all at one time.
Send your resume to a couple of agencies first, study their feed back, and modify your resume or other things. Then send your updated resume to a few more agencies, see what feed back they have on you, and make appropriate modifications of your stuff.
If you don't receive any response after sending these resume, you definitely need to change you resume or cover letter.
Remember, save the best agencies to the last. Wait until your resume becomes perfect, and you have gone through a few interviews with other agencies. Because you are expecting getting a job from these best agencies.