Hey there!
I really enjoy this nice game and can only imagine how much effort and time you have to put into it to make it this interesting.
Not sure if this is a bug, but my guess says it is…?
One little thing I noticed in the current build (1.31) is that the [Fit Window]-slot will start with 4 (5?) Servos in the stockpile once you have the necessery stuff researched (not sure which one it was… 
).
After 4 or 5 cars the local stockpile of the slot gets a bit strange and instead of 10 windows and 6 servos or even 8/8 the servos start stockpiling (12+) and leave no room for the windows.
This leads to “insufficent materials” for almost every car that comes through there which is not a lot of fun when building a factory optimized to 1 minute (or a bit less) per slot-type…
The stockpile thing next to it cannot move the windows fast enough over 3 squares to eliminate it even with [Predictive Stockpile] researched.
I am not sure if there are other slots that behave similar as I only confirmed this on the [Fit Windows]-slot so far.
The [Fit Wheels]-slot seems to do something similar too, but that is only a guess when looking at the pop-up notices and not confirmed through rebuilding it several times and taking a close enough look…
If I remember correct this wasn’t an issue in the earlier builds, but I can be mistaken on that…
If you want some screenshots i can try to get some, but not sure yet if I know hot to get that done…