I think more become available as you level up, rather than as an "a unlocks b that unlocks c" scenario. They have minimum levels attached so you may not have reached that level yet. They go up in 5 level increments with 2 hosts per level. You need to talk to the Battlewarden to unlock the later ones. If you're a Limsa person try the one in Swiftperch. The Ul'dah one is in Horizon & the Gridania one in Bentbranch Meadows.