“Always enter the conversation already taking place in the customer’s mind.”
When you are writing to customers are you positioning it from what matters to them or what is important to you? As Robert Collier stated, you need to enter the conversation taking place in their head already. When you write what is important to you it may not address the reader's concerns.
When writing to customers make sure that the words you use and the perspective you write from joins the conversation in their head.