For what it’s worth, the members of my local club all use charcoal soaked in parafin to start their fires. This seems to rapidly lead to the need for coal and even the larger locos seem to be in steam very quickly. Is it possible that your kindling ashes are clogging the grate. This is just what is required for a wood burning stove where top draught is needed but not good for a fire requiring strong bottom draught.
Chris.