Do you have the Automatic servicing option enabled in the GSX Settings ?
Also, since GSX now allows services to run at the same time, it's normal you might SEE a vehicle from one service to arrive before another one, even if you called them in reversed order, simply because they might take a different path to arrive so, for example, if the nearest Fuel parking is close, the refuel truck might arrive before the Catering truck and, of course, the baggage loaders will always arrive before anything else, since they start on the parking.
