The Sauromations have to attack the towns first, and since they usually do nothing, it is a rare mission to see. Also, the reinforcement army spawns as a rebel army that you have to bribe. Bribing the army was easy in RTW1.2, but CA increased the bribing difficultly somewhere along the way, and the army is much more difficult to bribe now. It may just be spawned under Pontos control in the future.