smoke on a calm day with range cooker

Hi, I have a Mulberry solid fuel range cooker which heats water and rads. On a windy day it works fine but on a calm day like today, when you open the door to load fuel smoke comes billowing out and there never seems to be much flame. The fact that on a windy day it works grand leads me to believe that I am not getting enough draught in the chimney and I am wondering if anyone knows of a product that I can put on top of the chimney that would improve the draft. I have a mulberry stove in my living room that works fine constantly. They are both connected into seperate chimneys and the flues have been cleaned regularly.
To my untrained eyes and ears it just the big range requires a good draught-no building or trees nearby