On my map ore and coal are close to each other. There are two cities not that far away which have industry nearby which can process ore and coal … so far so good.
I connected the mines with two separate lines with the cities as its destination. Each city has a truck line from the station to the factory and (for products) to a truck depot near the city centre.
Ticks from mines to cities are ~8-9 minutes and truck ticks are about 1 minute.
The mines start to produce (very slowly) while one mine has always slightly lower productivity. When the productivity reaches ~80 (or earlier) everything breaks, factories don’t produce, mines do not deliver any more. Productivity is 0 within a year.
You dont want finished goods to be in the center of the city. You want it to be close or in the center of the industrial area of the city. So they dont have to walk that far to the factories that need them. Sounds like you either are not having enough frequency when production is ramped up.