Off the top of my head:
Invalid encounter blueprints (blueprints not existing).
Encounter lifespan timer being low
Encounter blueprints mistakenly being set to go into stealth mode, so the pc can't see them
Encounter blueprints not using the encounter blueprint script
Hostile spawn waypoint missing
Hostile spawn waypoint in an invalid location
Changing the spawn percentage function so it doesn't use d100
Some other function like InitializeEncounter(oPC) was altered
Lack of valid locations around pc for encounters to be jumped into
Terrain trigger identifies it as wrong terrain type, causing wrong encounter table to be used
The terrain trigger isn't working but neutral encounters are working and are being spawned instead
The terrain trigger isn't working but plot/special encounters are working and are being spawned instead
Do you have these problems when you use the demo module as is?