The bus is coming but no passengers walking to the plane.
The common case of invisible passengers, at parking spots without a jetway (those with a jetway have different possible issues), is you reached the maximum number of objects in a scene, which is documented at 1000, usually caused by using AI Traffic injection programs but, on very dense airports it's possible to reach that limit even without it.
After the limit is reached, the sim won't accept creating new objects, resulting in passengers and other vehicles not to be seen anymore. The only solution is lower the number of objects in the sim, by reducing AI density and, in GSX, reducing the Passenger Density slider.
