Wood too close – Lacks sufficient air to sustain a fire, the temperature drops and it will go out. Blowing air into the cavity would keep the fire burning. If the pieces of wood were resting on green wood or a metal grate leaving an air space below the wood, an updraft might be present to help the wood burn.
Correct distance – Good balance of air and heat to let the fire burn. While the wood will burn very well the fact that it is burning will enlarge the spacing and you should stoke the fire so that the remaining wood will be pushed closer together or add additional fuel.
Too far apart – The heat is lost and the temperature is not high enough to sustain a fire. The fire will gradually go out.
