The Train Fever model of production is based on demand. This means that the factory will produce only if there is a demand for its product. In your case, it may happen that the city that get the goods is getting some of them from other sources and therefore ignoring your service.
You can turn the cargo view on and see if there are some packets that, like ants, are marching towards the city you are serving with your trains. In that case, that would be the reason why the factory produces as little as what you said.