I have a suggestion that should be improved:
The conveyor Lines could be improved in that way that
if a car is ready at the one slot and you have after that two slots of the same type
and the car should move to the slot which is just free.
You can see it on this picture what I mean:
It does that already, if the first slot is not open, it will go to the next if there is an open path, if both are full, it seems to queue at the closest open one.
This is a rather extreme example I made…
Looking at your image, it looks like your conveyor direction arrows may not be flowing the right way. That is more than likely your problem. I’ve done that alot when I first started out.