If by unique arts you mean arts like Gae Bolg or Omnistrike, the general requirements are these:
-The character that can use them MUST be the union leader. Basically this means Rush, David, Torgal, Emmy, Jager or Irina for the most part.
-You must have enough AP to use the art or summon.
-You must target the correct union. When your union's portrait shines yellow it means the Unique art command is available for one of the enemy unions on the battlefield, but not necceserely the one you're fighting right now.
-Some arts, especially summons, tend to come by more often when yout union is currently not deadlocked. This is mainly because using Gae Bolg, Cyclops, Namul Niram or Lob Omen is a one-person action, incapable of deadlocking and renders the other union members unable to act.