You must be sure both cargo doors are configured properly, and high enough for the loaders to be reached.
The lowest height the cargo loader can reach is 1.92 meters from ground so, if your door is lower than that, a Belt loader would normally appear (if you set "Auto" on the door config), but if you explicitly wanted an ULD container, and the door is too low, no vehicle will appear, since we haven't any ULD loader that can go lower than 1.92 meters at this time.