I don't know how much gasser experience you have but I have found that rounds do not work in gassers. Basically you need a coal bed with a load of wood on top of that coal bed with full contact with the coal bed. So if you have rounds split them in half and place them flat side down onto the coal bed. Then on top of that pack your wood light tightly together with varying sizes splits as tight together as you can. This will allow the gassing process to continue as each layer of wood is broken down to coals.
This is my first gasser, so not a lot of experience. The loading method you explained makes a lot of sense and I think it explains at least some of the issues I'm having. Looking at the firebox a few hours after loading, I'm seeing a 6-8" bridge over the nozzle, where the hot coals have burned away (probably from too much air). Sounds like I need hot coals directly above the nozzle in order to get it gassing well.
My plan is to install a variable speed control this afternoon and close down the intake pipe a bit. Tweaking the air settings should be easier after that.
I'm curious, what size rounders do you generally not bother with splitting?