The material was poorly written, badly designed, and poorly tested. Test your letters before you roll them out on a large scale. If you're getting the kind of response you want, mail larger numbers. Enclose a response form to increase your response. Remail the same letter to the same people two or three times.
Repetition often helps.
If the letter is persuasive enough, it will get through. Give yourself time You can't expect to produce an exceptional document overnight. In letter writing, you start with nothing. Letters can be about anything. That's why they're so difficult.