Greetings, I installed a Jotul GF 400 Sebago, gas stove last winter and it was working great. This winter at some point soot is building up on the glass and it does not appear to be generating nearly the heat it was last winter. I have cleaned out the stove as best as possible. I also noticed that the flames are now longer yellow flames, possible air shutter issue. Any suggestions or recommendations gladly accepted, thx