I tried posting this last night but the site was being stupid and I was at work and had to actually go work. Not sure if you've figured it out yet, but here's what I was typing for you...
One of the first things to check is to make sure the fiber seal is in place between the ask auger tube block off plate and the tube and that the thumb screws are tight. Next, make sure the door that when the outer door closes, it completely closes the little block off door that leads to the airbox. Mine was warped when I first got it and it needed a little persuasion to get back straight so it sealed up completely with the outer door closed. Also, remove the blower from the ash tube and make sure that the door for the blower swings freely and will close when the blower is off. If that sticks open, which it's easy to do if it's at all rusty, your unit will pull a natural draft and keep heating even if the blower is off. If you have an extended chimney installed, the extra height can also force a greater draft strong enough to pull the blower door open and allow air into the firebox.