As explained many other times in the forum (and in the manual too ), the ground operator is not selected depending on your own airplane airline, but depends on the airport and on the parking code of the parking you selected.
This is made to prevent unrealistic situations where you might be served by your own operator in every part of the world (such as having a Lufthansa crew serving your airplane when parked in Japan...).
If you read the manual or (even better) the PaintKit documentation, you'll see how the scoring system works.
The next GSX update will allow you to select the ground operator from a list of SUITABLE ones but still, it will be only possible to select between all the suitable candidates for that airport.
By reading the Painkit documentation, you might understand how the system works and, eventually, tweak the SIM.CFG of the affected vehicles, to have them appearing in airport you chose.
