In order for a smeltery to work, a few conditions must be met.
1.)There has to be a seared brick floor inside of the smeltery.
2.)The entire thing must be comprised of seared brick, seared glass, or the functional pieces of the smeltery. (Drain, Smeltery Controller, etc.)
3.)It must remain fueled.
4.)All ores to be melted must be input through the smeltery controller, not manually placed inside the smeltery. You are able to throw items in, but It may not always detect it
As long as these conditions are met, there should be no problem.
*Edit: Also, do not manually break the blocks that show up inside the smeltery when you place them in through the smeltery controller. This can break your smeltery and cause glitches.