My one-year-old wood gasification boiler seems to have the passage between the firebox and the refractory chamber blocked. The wood burns in the upper chamber, but there is no flame appearing in the refractory, and no ash has been deposited there for a couple of days. Probing with a hooked poker among the coals to find the passage (and I have seen it often in the past while reloading) hasn't done anything to open it up. Any ideas?