The GoT Mod uses a "Megawar" mechanic that, whenever the Iron Throne empire breaks down with internal strife and warfare, allows the vassals to choose between remaining neutral, actively support or oppose the throne or the rebels, or sometimes try and declare independence. This means that every time a major war starts the throne will split into the different kingdoms, and the Lords Paramount will take a stand on whether to help you out or fight against you, or just not fight at all. When the war is over they will reunite and become your vassals again - unless they fought for independence and won.
It is likely that the Paramounts of the Westerlands and the Reach does not like you very much, given that you were just at war with them as well, and thus, they aren't very keen on helping you out against the North.