Most likely, you have too high AI settings, coupled with a too dense airport, so you surpassed the maximum limit of objects in a scene (officially documented as 1000), after which Simconnect no longer responds correctly to commands, causing all sort of malfunctions.
The easiest way to reach the limit is to use AI traffic, especially with AI Injection. But it IS possible to reach it even without it, if the airport is detailed enough.
What you CAN do from GSX to mitigate the problem ( not fixing it, only Asobo can really fix it, by allowing more than 1000 Simobject in a scene ), is:
- Disable the "Ground Clutter" option in GSX. This will only be effective for default airports that had their jetways replaced by GSX.
- Lower the Passenger Density slider in GSX. This will reduce the number of passengers visible at any given time, which will affect the overall number of Simobjects in a scene.
But the most effective way to deal with the Simobject limit, is reducing the AI Density. If the AI traffic product has an option to disable Ground Services for AI, that's the most effective way, because each AI could generate extra 5-6 objects as ground services so, 100 AI airplanes could add 500-600 Simbojects and, if you add the airport objects, and the Jetways, it's very easy to surpass the max Simobject limit.