I have noticed that when you try to use a blast furnace with hoppers on top and/or below the blast furnace, it doesn't seem to work.

The hoppers put the blocks into the blast furnace, but the blast furnace doesn't fire up, or begin to smelt the materials inside, even with coal and the other item inside.

Me and my friend noticed it while attaching a chest to 2 blast furnaces using two hoppers to connect to the individual blast furnaces. From there, we had hoppers connect from the blast furnaces to another chest.