POD #1: The Spanish navy based at Montevideo defeats the Argentinean Navy in the independence war, securing naval superiority in the River Plate and mantaining their foothold in Montevideo.
POD #2 (optional): Jose de San Martin dies in the battle of San Lorenzo, thus the Argentinean government doesn't even consider (and reject around that time) an offensive into Chile and continues the useless efforts in the northern front (Alto Peru, current Bolivia)
POD #3: The Spanish manage to assemble a fleet and fair number of artillery pieces after Waterloo and set sail to Buenos Aires. Upon reaching the city, they disembark not with the purpose of conquering the area, but to destroy the city. Difficult, but with fronts in the east, north, west and south (the indians), Argentinean reinforcements aren't arriving in time to prevent the destruction and the death of most inhabitants. After all, the Spanish troops aren't staying, they're moving to Montevideo afterwards. They score heavy sucess against revolutionary troops in current Uruguay, breaking the resistance in the River Plate. Further sucessful campaigns from Chile and Alto Peru can finish the war in the southern cone, and the Spanish then do their best to cripple the area, as they don't have the manpower to effectively consolidate it.
This means Bolivar will have an easier time in the north, eventually driving the Spanish from Peru and pushing south. Around the late 1820's Paraguay launches an offensive towards their south, helping Bolivar's campaign, thus annexing the current Argentinean provinces of Corrientes and Entre Rios.
Still, very unlikely.