Everything in your letter which is done well is calculated to increase response, obviously. A weak opening will get you fewer replies, as will poor writing. But there are certainly three elements often overlooked which make a lot of difference. If you have missed out any sensible reason for response, you are going to get fewer replies.
Credibility is a major problem with many letters.
Just because you think it's wonderful, that doesn't mean to say your reader does. You're writing to somebody who has never seen you and doesn't know you and is probably fairly sceptical, so make sure that you have adequate proof and testimonials. And, perhaps the biggest mistake of all is not to ask insistently enough for the order. It seems an awful shame to spend time, effort and energy writing a persuasive letter without asking for the order as strongly as possible. This means re-emphasising the benefits that will be obtained, underlining what will be missed if the reader doesn't reply, reminding that reader of any incentive you're offering, and of course making it easy to reply.