Why isn't my paper mill being provided with lumber?

I built a Paper Mill awhile ago (top left structure in the below image, over the river), and I just noticed that it's not being provided with any lumber. As you can see from my image it's within the radius of the market building (bottom right), but it's not being provided with lumber. I've verified that the market is reachable by road, and I've got over 100 lumber in storage.