Source industry producing almost nothing?

I have a forest pretty close to a sawmill here. Gave them both truck stops and made sure they were in catchment range. Assigned some trucks. That kicked the forest into production, but interestingly, it is making wood at an absolutely glacial pace. Each truck has to spend months waiting just for a single unit of wood. It does not appear to be ramping up after several years of waiting. What could be the cause of this?

I do have another line with a forest bringing wood to that same sawmill, if it makes any difference. The sawmill also isn't connected to any output currently and the other line works just fine. Is it perhaps that the sawmill can only handle a cumulative input rate of 400?