Whenever I call the deboarding vehicels, they never get finished, which means, the catering trucks won't appear at all
That doesn't happen here, I can surely ask a deboarding and catering immediately after, and catering vehicles comes normally when deboarding is finished.
Are you using the most current GSX version or, if you haven't installed the latest version, are you fully updated, including the optional vehicles download ? If you installed GSX with the current installer, the optional liveries are already included, but if you installed an older version and get all the Live Updates, you'll have the download the updated liveries for vehicles separately, because they don't come as Live updates, and there were a couple of missing liveries for the catering vehicles that could create exactly that problem, with catering not coming.
PS: Why did you guys put the limitation in that you gotta wait with ordering catering until baggage service has finished, I mean in real life, those are two different tasks and no one there has to wait for each other, at least not it that hard way
In real life there is a bit of overlap, but not much, especially between services involving passengers, see here the diagram on page 6:
http://ardent.mit.edu/airports/ASP_exercises/ASP%20Zerbib%20Ground%20Handling.pdfAs you can see, different services do overlap, but not really with passengers boarding/deboarding, there's just a very small overlap between deboarding and the start of cabin service, and no overlap at all with boarding so, GSX simulates this fairly realistic.
On top of that, there's an added complication that, if we allowed too many services at the same time, there might be issues of vehicles running into each other when arriving/returning.