Is there insulation around the feed tube, burn tunnel and riser? When affirmative, what is the space between the insulation and the barrel? Brick isn't insulative at all, so what's the nature of the insulation?
What is the ratio of feed, tunnel and riser? Is this at the very least 1 : 2 : 3.5 measured along the centerline?
Is the cross section area the same throughout with a lot of space (typical 200% of system size) in the transition area from barrel to horizontal flue?
Is there a vertical chimney (preferably insulated) coupled to the end of the horizontal flue which does reach all the way above the highest point of the roof?
All these are requirements, the J-tube
rocket mass heater is an extreme kind of heater, it need to be done just right when you want it to work right out of the box.
Nearly forgot: your riser and feed seems to be dry stacked. This isn't any good, all the crevices, however small, need to be sealed.