Five years ago we purchased a Vermont Defiant. After years of enjoying it, the stove now burns less well. In the non-catalytic mode, the heater always fired better than in the catalytic mode, but now there is no difference anymore and it seems the stove is only burning in catalytic mode (although the damper is open). In other words: there is no difference in both positions. What could be the issue here? Kind regards, Michiel, Netherlands.