Assuming you’re not on the shore and water is rising over it, the likely issue is the smoke coming out the top. If there’s too much, it snuffs out the flame, just like it does for a campfire. You need to build it outdoors or make sure you have a chimney to vent the smoke out.

Smelters produce a ton of smoke. You need a vent.
https://steamcommunity.com/sharedfiles/filedetails/?id=2819643017
Like this

like everyone else, i guarantee its a smoke issue. i ran into the same problem even though i thought i had enough ventilation. I added just a bit more ventilation and problem gone.

Neat thing is that you can place them so they clip through the roof slightly and, if done properly, the smoke generates above the roof and goes straight outside without worry. Exact placement for this is tricky though.

looks like too much smoke accumulated/cant travel up effective enough

Just like fires, the smelter and coal kiln need ventilation above them (a place for smoke to go) otherwise they become "smothered" and will flicker on and off.

I usually build my main base underneath Elders altar, which means i cannot vent the smoke out so I build a false ceiling to allow the smoke to rise up and eventually dissapate safely in the space above.

https://steamcommunity.com/sharedfiles/filedetails/?id=2657677489

https://steamcommunity.com/sharedfiles/filedetails/?id=2434836833

Indoor smelters require high roofs otherwise they can turn on and off at times.

edit: The Forge can provide a comfort bonus when you are in it's menu but it needs to follow similar rules when it comes to places of comfort. Basically a large enough roof above the forge and some walls should be fine.

smelter

Originally posted by aaronrt92:

smelter

Post a pic of your setup. It would help a lot.

I thought it just lit up when you used it.

It does, but as with all fires, it needs an exhaust vent so it doesn't suffocate. My smelters are all outside, so I don't have that problem.

Edit: Ok, maybe there's a misunderstanding. Are we talking about the workbench forge, or about the smelter?

You must have something above it that is blocking the smoke.