I have two areas on my map that are producing fish. The distance between them is several tiles. Both areas have their own Fish Factories. Both areas have cargo harbors to export their commercial goods. There are no other connections between these two areas.

But all the Fish Factories in one of the areas simply REFUSE to use their own local source of fish. Not a single truck will come from local fish producers. They are ALL coming via the cargo harbor. This in turn means it takes forever to get new supplies of fish. The factories will remain in "Not operating" status for 99% of the time while they wait for the arriving cargo ships.

I've spent tens of hours now trying to figure this out. District Restrictions does noting at all. Building Restrictions does nothing at all. Distance Restrictions does nothing... at all. I've even tried rebuilding all the factories and Fishing Harbors/Farms.

I'm not using the Fish Warehousing mod, so all fish transfers are direct.

Got any tips for figuring out this? Or is there something very wrong with how fish is handled.