There are lengthy threads on this in the past. You may be able to increase your chances of ch.9 by asking the crew, but this won't work every time. Like others, I've found ch.9 working on most post-9/11 flights. Sometimes, it'll be on but your headset may be inop, so don't eliminate that possibility. I did find a senior Captain last week who was outspokenly anti-ch.9 because he "didn't want anyone who might cause trouble on his aircraft to know more about its position" than he did. While I disagree with his reasoning, he was the Captain; and as UA says, it's "at the Captain's discretion"