vanishing points of interest

While I'm adding areas and places to my map, some POIs (points of interest) are showing up without a problem, but others aren't appearing. I've checked, and the POIs have been created, but they aren't showing on the map or in-game when I test to see if they're working. I've tried deleting and redoing them, but the same thing happens. I haven't noticed this problem when creating areas, only with POIs.