Linda,
I have a 6 burner NC gas forge that has two banks of 3 burners. My primary set had issues on the front burner. The problem I had was that some debris had gotten stuck in the gas orifice in the distribution tube.

To fix it I had to unbolt the gas assembly that feeds into the top of the burner so I could take a set of oxy-acetylene tip files and clear what was plugging the orifice. A critter such as a spider might have gotten up in the burner and spun a web or a small piece of rust or dust might have fed into the burner orifice from the back.

The sides and floor of my forge are fire brick but the top of the forge is a composite ceramic fiber board. Mine is cracking but doesn't need replacement yet. When I talked to NC forge about it - when I was getting a spec sheet for my fire marshal review - they said they could cut me a new piece if I sent the model number and dimensions since my model was never in formal production. It was an early one I got at Madison in 2003.

Added 2 minutes later:

Also - you can get a piece of kiln shelf (about a 5/8" thick) from a ceramics supply store to place in the forge as a patch if the fire brick is gouged and you don't want to do a full replacement.