she looks a little loose from the pictures. does it leak alot around those doors? it could be that you are leaking your heat out in so many places that the chimney cant create a draft and pull the heat all the way through your smoke chamber.
my cheap offset has pretty thin steel (about the same as your drums) i have used old wool army blankets on the top to help hold the heat.
COS, SS gasser,10-10-10 UDS,and 4-4-11 her twin sister, 22.5 silver, and others.