A common problem I hear from those trying to help agile take hold or grow in their company is that outside individuals or groups that support is need from do not try or even want to help. "How do I convince them? Force them? Go around them?"
My experience with change is that well-paced, lasting change comes through people who want some part of the hoped-for outcome. They may not want the change itself at all. Heck, change is hard.
So why do some of us simply expect people to do the "right thing?" As trite as it sounds, "What's in it for me?" is a fair question, if not just plain reality. How is adopting agile going to help them, their department? How is it the best solution to for current or future problems? How will it bring at least some success with little risk? How will it help their careers in 1 or 2 years? How will their boss view their decisions, and help their boss with the same questions above? Based on personality, how does going agile help them with personal decisions, teamwork, consistency or processes?
Sometimes when we believe in something strongly, we can forget that others _can't_ see what we see. We forget the process we went through before we became convinced, didn't read the "A-ha!" moment article or hear the great speaker at the conference. Others don't have the same experiences we had of painful projects, missed expectations, resource problems, or failures we felt responsible for. Often, these other groups haven't been responsible for anything except providing resources from their department in a predictable manner to many demanding and impatient internal customers.
Meet these people where they are in their workaday, pressure-filled world. Hear them out. Ask them what their problems are. If it seems the right time, share with them how agile will help those problems while not exposing more risk, unknowns and change than would make it worth it. You may have to wait a week between asking them and sharing your thoughts. If they have questions, do the homework necessary to provide answers succinctly and in a what that can be easily shared with those they work with. Ask them what success would look like to them if they were to try agile. Work with them to plan the next few steps.
Patience and consistency go a long way, and if they believe you are looking out for their good, they'll begin to trust you at a level that helps everyone move forward.