One of my first air ports had had a long line of small turnaround stands connected to 2 medium ones. the baggage bay connected to the medium stands but because there wasn’t a actually service road it couldn’t find a path to the medium stands, even though my fuel trucks where able to get the them.
Can you show us a photo or sceenshot of your airport?
I think the problem is more likely about baggage system than roads. Because roads seem fine and since tanker find its way to stand, baggage truck can also fnd its way.
Then many things in baggage system’s itself can cause it.
Is the connecting service road labled as secure zone?