When you approach a passive candidate, you need to be sure you are offering them something that will be mutually beneficial. Messages that are all about the qualifications you need from a candidate usually go unread. Messages that are all about what you have to offer also usually go unread. Messages that strike a balance between what you have to offer and what people will bring to the table will help a prospect visualize themselves in your organization and visualize how their work will connect in a meaningful way to the mission of the company.
When you reach out to a passive candidate, your message has to express that what you have to offer will give them more than what they already have in some meaningful way or there will be no reason for them to respond.