Did you by any chance remove the burn grate and clean out any fines that may have fallen into it? As you know, there is not much air required to keep the fire alive, but frequently in the inclined bed stokers some folks forget to gently remove the grate and clean it out. It's a great idea a couple times a season if you have lots of fines in your coal supply. If your open window policy works well, then perhaps you might install a permanent outdoor air supply tube for the stoker. Keep us posted, please. Obviously, the hot weather isn't enhancing the draft, so you have a few things to check out.