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. The class focused on writing query letters to publishers to get a book contract. The course lasted six weeks and met for two hours each week. Lots of work for just one letter. I mailed the letter to about 30 publishers and got 13 responses.