Have you reloaded the airplane and/or restarted Couatl ? The changes are not read immediately, only if the airplane is reloaded.
In any case, I've checked another thing and, of course, there's always a reason: the loader platform minimum height is 1.95 meters (this is absolutely accurate, from the loader actual user's manual...), it can't go lower than that.
The cargo doors of the 737 NGX are set at 1.35 and 1.65 meters so, there's no way the loader could service them, which is why you don't see it even if you ask for it in the GSX.CFG file.
As I've said, you are trying to do something unrealistic: there's a reason why the real 737 doesn't use containers, the door is both too small and is too low too.