From your report, it seems the airplane door is too low to be reachable for any of our baggage loaders. I think the minimum height is about 90 cm.
If the door is less than the minimum reachable by any of the vehicles, GSX will use a "backup" strategy of showing just the truck, passing close to the airplane, then going away, so you will know that something happened and GSX was working, but it could find a suitable vehicle.
If the door's height it not too far from being over the minimum height, I guess you might "cheat" a bit, and raise it a few centimeters, until it's higher than the minimum height reachable by the loader. You can double check the correct number by looking in the [Couatl] section of the SIM.CFG file of the FSDT_Baggage_Loader vehicle, it should be something like 0.90.
Note that, if the door is set exactly at the minimum height, it might still not always work, depending of how much the plane weights, which will alter its actual height from the ground so, it's best to give it some headroom.