If you don't get gasification and just burn wood, you may have too much air going in. The process relies on enough air to start the fire and heat the fuel enough (750°C +) to release volatiles (CO, H2, CO2, CH4), but the process should be most near anaerobic.
Too much air combusts the carbon in the wood structure, rather than leaving a residue of intact black char in the bottom. Looks like Charlotte's stove is spot on; fuel packing may be critical to a good performance (one may need a stove meet or competition with more stoves, fuel types and beer to find out).
UEA in Norwich have one of these running on woodchips and powering a CHP plant. It's a little bigger.