I know the boarding and catering are triggered since I hear the GSX voice state that.
Then you know GSX is working and it started the service, so your problem might be only visual. For example, if you have AutoFPS utility, it's possible it's changing the Objects LOD settings without you realizing it, so the objects are there, buy they won't be seen, because the LOD went far too down.
Another possible reason is having exceeded the maximum number of Simobjects in the sim, usually due to too high AI traffic and/or too dense airports. The GSX manual has a chapter dedicated to this, with all possible suggestions to reduce GSX own contribution to the number of objects, and explanations why it's not the only one contributing to it. It's at Page 96.
Lastly, a change/problem in the GSX airplane profile might cause this, like having some or all doors not set correctly, resulting in no vehicle being found for that door configuration (usually heights too low or not set at all)