There's a known problem in the simulator itself, which allows a maximum number of 1000 SimObjects in a scene. This includes jetways, AI airplanes and Ground vehicles and, if you use 3rd AI traffic, on big airports like KLAX, it's extremely easy to reach this limit, which will result in empty airports.
Every add-on you have can contribute to this limit so, until the problem is fixed by Asobo and the limit would be eventually raised, your only options are work on all add-ons involved to reduce the amount of objects, which means:
- Reduce the AI density in whatever 3rd party AI product you are using.
- Reduce the GSX number of objects by Disabling the "Ground Clutter" option in the Config Panel of the FSDT Installer and run an Update again, so it will download a lightweight version of the jetway replacement files that won't contain anything other than Jetways, thus not increasing the number of objects compared to the file they replaced.