I have a gas fireplace model Archgard Optima 45. Pilot works great and stays on when doors are open or closed. Main burner comes on great when door is open. soon as the door is closed the flame starts going out and over 10 seconds eventually goes out completely. If i leave door open flame is full and keeps going. I took off the complete pipe and vent pipes from top of fireplace to ensure that no exhaust was getting blocked. even with the pipes completely off the flame still goes out when door is closed.
You need to have a way for air to get IN to the fireplace as well as a flue for the gasses to get out. If the front and door is sealed up too tight, the flame consumes all of the oxygen in the fireplace and extinguishes itself. Usually there is a little vent hole or louver in the front that draws air into the fireplace, or there is a pipe coming from the outside supplying air. If that pipe got blocked by something, that would do it.