I just recently discovered a method of using a conveyor carousel in between slots to evenly-ish distribute cars to theee following slots.
In the attached pictures there are two of these carousel / snake layouts: the one after the chassis assembly works fine, but the one after fit body before paint stopped working once i added 2-3 more paintjobs.
Cars just go in circles but only occasionally enter the paintjobs let alone file the waiting-spots behind them. Rebuilding all paintjobs, the conveyors and the fit body slots didnt help, even shorter conveyor carousels dont work any more.
I have had this happen also. What i find is that if you put more than 8 units of the same slot in a row - for some reason the cars bypass the first available slot and always want to move to the end slots.
You can fix this if you put 5 slots of paint on one side of the conveyor - and then 5 slots on the other side of the conveyor. Then the cars fill evenly and you dont have to snake the cars around.
You just have to wrap one of the exits sides back around to the next slot. Or put them 90 degrees from the beginning and ending slot.