When coming from a save it is showing that the next required line is not available as clearly seen it is there, now if that line runs that car will not move but others after a wait will pass under that car. I am running 4 lines plus almost all the “makes” items plus 3 of each of the advanced “makes” and shy only a handful of I can wait on upgrades. My factory runs fine and making money finally 
though I am still learning
the balancing which is my self learning curve and still no issues till after coming back from a save.
Can you provide screenshots and the save games?? It’s difficult to see the issue with just a description.
Sounds like I’m having a similar issue.
Attached are two pictures, debug, and my saved game file. https://drive.google.com/open?id=1oZb08eYIXR4uPR0MEmrX7X3ce26xYw0c
Version A 1.41.
The second screenshot seems to show a slot between where it’s asking for the fit steering wheel and the actual fit steering wheel. Which slot is that?