Production only begins to accumulate if factories in towns increase their demand for the resources and if they get delivered under the arbitrary time limit of 20 minutes travel-time.
you need to therefore supply the resources fully from logging camp to the saw And from the saw to the town factories because (unless youre lucky) cargo itself travels too slow from the mill to the factories to come under 20 minute arbitrary rule, which means that you have to supply the whole goods chain with your preffered transport system so that goods get transported in time.