This information is false and people keep repeating it for some reason. If you have 2 followers at 645 and all other followers 630 or lower your highmaul mission wont appear. I had 2 645 followers for over a week without spawning the highmaul mission, 30 seconds after I upgraded my 3rd 645 follower I got the mission. For a friend of me the highmaul mission only appaeared when he had 2 645 followers and made his 3rd followers 638. Anyway, the trigger is somewhere between your 2nd and 3rd follower getting 645, just 2 645 followers wont do.