As I understand the manual, the bounding box has a size of both trigger ranges plus the bounding box of the triggered entity. If both entities are within that bounding box, the event will be triggered. Additionally, both entities need to be in the same level sector. A level sector is 1/32 the level size, so on very small levels this might affect triggering also. Make your level large enough so that 1/32 is bigger than your trigger bounding box.