I have this configuration:
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Now, these furnaces get their input from the left (either iron plates, stone, iron or copper), and get coal from the right side. They also put their output out on the right side (steel plates, iron plates, copper plates or stone bricks).
The issue I am facing is that some of these furnaces stop working after a while, and are left in a conflicted state. When I then open these furnaces, they look like this:
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They clearly have fuel, they clearly have an input resource, but won't produce any output. It's not because the resulting transport belt is congested. If I shift-click the input resource of the furnace to take it back into my inventory, the inserters suddenly start working again, and insert stuff into the furnace... For a while. During this period, the furnaces also work just fine and put their output on the belt as they should.
If I insert MORE of the conflicted resource into the furnace than there already is, the furnaces also start working again... For a while. Again, this time, furnaces also work as expected and put their resources on the output belt.