It is really hard to find any detailed history of Central America anywhere. I got interested in this because my wife is from Honduras. The problem is that it always has been essentially a backwater and relatively ignored except for bananas, stopping Communism, and the Panama Canal (and Panama was considered part of South America until the US got it independence).
The problems were already there from the beginning. Central America got its independence by default when a Mexican army came in and said it was now independent and part of Mexico. Mexico couldn't be bothered to hold it down and Guatemala, El Salvador, Honduras, Nicaragua, and Costa Rica left to form UPCA. Chiapas was part of the Real Audiencia of Guatemala but decided to stay part of Mexico. So without a common independence struggle, there was already a tendency towards independence.
Despite that, they set up a federal constitution. Borders were adjusted between provinces to make things fairer. Guatemala dominated in population so they split off Los Altos to make representation fairer, but still it was Guatemala dependent.
And like elsewhere in Latin America you had a split between Liberals and Conservatives. Francisco Morazan emerged as the champion of the Liberals and Jose del Valle emerged as the champion of the Conservatives. Both could work together. Morazan won the first election, had a decent term, then del Valle won the second. But del Valle died before he could take office and Morazan took the presidency again by default.
The Liberals had too ambitious of a plan to change Central America by trying to push for as much of a separation of Church and State as possible, pushing economic reforms, and trying to separate native tribes from any sort of Spanish historical tributary ties to the land. But the latter part really backfired as the natives treated it as another attempt by the Europeans to trick them into stealing their land. Combine that with a cholera outbreak in Guatemala, ineffectiveness of the federal government to help, and local priests blaming the liberals for causing it to happen, plus a very charismatic mestizo pig farmer named Rafael Carrera who ironically supports Conservative Criollo policies, and you get a guerrilla war that takes down the Federal government. The provinces go their separate ways, several halfhearted attempts to restore the union fail, and Carrera's dictatorship in populous Guatemala props up Conservative leaning governments in all other Central American countries until the 1870s or so.
One PoD is to have the Central American countries more involved in fighting for their independence. Staying part of Mexico was another proposed PoD, but that makes it part of Mexico and not UPCA. Another is to butterfly away Rafael Carrera and/or the cholera outbreak, but that probably just buys time, not prevents the breakup.
IMO, the best PoD involves del Valle living and having peaceful transitions of power between the Conservative and Liberal factions. For the Americans on the board who are familiar with Early US History, it would be basically the same situation as if both Jefferson and Burr died or had something that prevented them from taking office again and Adams came in by default. Then if del Valle doesn't try to become a dictator himself and passes power on, to especially a Liberal, then UPCA has a chance with a tradition of peaceful power changes.