They blew their load with the red wedding. It worked because it was the consequence of several important factors of feudal politics and logistics. A boy-king needed to get his army across a river, so be brokered a deal with the lord who owned the only viable crossing, a man from a house that was a rival of the boy-king's maternal bloodline. The boy-king's highest military advisor was also a rival of his paternal bloodline. The boy-king reneged on his deal with the lord of the crossing, so the lord they were at war with offered a deal to the boy-king's rivals, betray the boy-king, and the war will be ended with the lords not only being given amnesty, but will be given control of the boy-king's former holdings. And so they did.

Compare to later seasons, where armies just magically appear wherever and whenever the plot requires, and characters can break their word and do whatever they want without any consequences. Cersei blowing up the great sept should have sparked a peasant revolt unlike Westeros had ever seen, but I don't recall if it was even mentioned after the fact.