This has been discussed and answered so many times, it's because GSX couldn't detect the jetway properly. This to prevent to see passengers in the air, which is what would happen, if GSX didn't checked if a jetway is connected properly before calling passengers.
When this happens, GSX will give you abundantly clear messages WHY the passenger boarding will only be simulated because of that, with different messages in case the jetway didn't connect at all, or connected partially. However, if you use GSX in the wrong way, by closing its menu with a click on the Toolbar icon making it Inactive, you'll MISS all text messages from GSX. If you are using the menu this way, please read the manual pages 85-86, where the correct usage of the menu is explained.
If you see the jetway visually connected, but you still see passenger boarding/deboarding only simulated, there might be many possible reasons for this:
- You are using a 3rd party airport but haven't disabled the GSX jetway replacement files for it. Please read the manual at Page 7, which explains in detail why you must do that, when, and why some sceneries won't be recognized automatically.
- There might not be a conflict but, because of too many objects in the scene, Simconnect might have start to misbehave, causing GSX not getting data about the airport, so it doesn't know where the jetways are (or even if there are any), so it doesn't show passengers, because it doesn't know there's a jetway there. This is caused by having hit the maximum limit of Simobjects in the simulator, due to too high AI density settings, too high GSX Passenger Density settings, or just too many objects in a scenery.