I attempted to customize it for my default B737
You don't say the most important thing: does the problem happened *before* you start customizing it, or only *after* ? The default 737 doesn't need any customization to begin with but, of course, it can customized. However, to understand the problem, it's important to know if the problem happens only after you start to customize an already supported airplane.
All other figures and all the other vehicles are present, just no baggage belt or loader...Thanks for any help you can provide.
It's not clear if you *really* have the SAME problem reported by lehbird, which is:
The luggage tractor simply pulls up to the plane, stops for a peirod of time, and then drives away
If this is what is happening, it's possible there's no problem to begin with and the loader simply doesn't show up, because you are loading zero passengers (no load on your stations), so GSX simply skips the vehicle.
finally I removed and reinstalled Addon Manager
If you are missing the loaded vehicles, because you skipped a major update in the past, doing this won't fix anything, because updated vehicles are either in the vehicles update installer (link as a stick thread on this forum) OR in the full GSX installer.
Try to download and install the full GSX installer instead, at least you would be sure you have all the most current version of the vehicles.