I was following a Super Smelter Tutorial by Mumbo Jumbo on YouTube – https://youtu.be/yyyhxRztamE

As you will notice in his video, there is a furnace at 04:06 that is non-functional. The video was released about a year ago, April 14, 2017. I am currently playing 1.12.2 Minecraft on my computer and followed this tutorial and have the same issue – one specific furnace will not get any smeltable items in it, but it will load coal into it.