I am also struggling with the pressure issue. As long flames are visible, i have a nice draft and sucking sound. After a few minutes of mere smoldering it smokes back. Already rebuilt 3 times, minor fixes not counted. By first and second version (now seen as mock ups only) was the heat riser too narrow (4"), making it longer (from 30 to 39") and raising the barrel accordingly didn't help either. I was thinking also, the internal heat riser enclosure drum, as per the Ewans book is too big, restricting flow. So i tore down everything, took more flue pipe and made a 6" diameter, 63" heat riser with bricks on 4 sides, the 200L barrel standing quite high up, on a waist high brick edge. Heat riser top end to barrel gap measured very meticulously, in the current version first 1,5", now increased to just over 2". It was needed, as it seems to me the upper end of the barrel tends to bulge in when hot. The vertical burning channel is now 20" long and 12cm by 12.5cm (23.25sq.in), and want to keep it that way.
What worries me most is the possibly inproper heat balance and the whole chunk of material having the same temp inside the barrel (no shock drop in gas temperature). The insulation in the top half of the heat riser is only 1" clay-perlite with that masonry enclosure (bricks standing on their side, after the first experience i was worrying over spacing toward outer barel). The 20 foot long 6" flue pipe was barely getting hand warm in mid section, so i took out 2x 90deg elbows and 8 foot of the piping. That helped somewhat on the overall function.
As still no foolproof firing possible, i want now to take out the perite clay insulation, tear down the whole heat riser, replace the outer enclosure with a 12"diameter and 68" long pipe from the scrapyard. The inside of the heat riser will remain the 63" long 6" flue pipe. The insulation will be dry (no clay added) perlite, to my calculation the remaining 100L bag will fill the space up. I also want to make a fresh air intake channel, because it needs now a close-by window being open finger wide to have decent burning.
I have made photos from all steps, including the dead ends.