Board Thread:Questions and Answers/@comment-27149960-20160622050931/@comment-28905095-20160702132730

Three cases for you to look into:

1) Make sure that the hopper actually goes into the top of the furnace. That is, look down on the furnace whilst crouching/sneaking and then place the hopper.

2) Make sure you only put meltable items in the hopper. If the furnace has preloaded something not smeltable, you'll have to clear out the non-smeltable item manually.

3) If no fuel (or no removal of previous smelted item (of different kind)), it will also seem like the hopper on top is not feeding correctly.

Hope this helps!