Have you done a co2 check also check the air flow into the unit and also the flue. Do you have an automatic damper on it if so then lock it in the open position and then fire the stove. If this does not work then it sounds like you gas valve is going bad. How old it the furnace and the piping and do you know the pressure going into the unit if its propane go tap the regulator sometimes in this hi lo weather you can get water that freezes in the line this happened to me on my gas fireplace.