I concur. Cold stove pipes dont draw very well.On a cold startup they have less draw, once the chimney and pipe gets warmed up they draw better, that's why they will puff back more when they are cold. The blockage affects it more cold as the draw is worse then to begin with. I would check the pipe and chimney to see if there is a buildup somewhere. Crumplimg up some newspaper and lighting it tends to get some warm air up the chimney and get them drawing. Just my 2 cents.