I have 7 assassins that have reached the rank of master assassin but I didn't get the trophy. My dens are full so I can't rank anyone else up. What should I do?
All 7 den leaders must be level 15 (with a little assassin logo under their name in the Mediterranean defense menu) to unlock this trophy/achievement. To get them all to level 15, they must have enough experience, and you must do all of the Master Assassin missions. The first of each den is unlocked when assigning an assassin as a den leader, but as they gain enough experience to become a Master, a second one will be unlocked. You can see if these missions are available by looking at the map for black squares with the assassin logo in it.
I think you are confusing assigning den leaders with reaching the rank of master assassin. Let me know if I am wrong. If so, it must be a bug.