This vertical smoker in my first of this type, I've used bullet types and had no issues like this.
My first use was yesterday and it started off pretty OK at ~225. The top door as expected leaked. Anticipating this I have door seal on order but it won't arrive for a few days. I also ordered a small grate for the bottom of the pan to keep the coals out of the ash.
So I stuffed the gap with some thick cotton which stopped the leaks fine, but after a while I had trouble keeping it at 160 while trying to vary the lower two vent openings, but the charcoal was burning pretty fast and I kept replacing it. The temperature didn't seem to change much regardless whether the vents were closed, or any other open position from 1/4 to fully open top and bottom.
Anyway, after a few hours I pulled out the charcoal pan as most of the charcoal was spent even though I had been replacing it, but the ash was pretty thick smothering the live coals that were left.
So I replaced the pan with another that was stoked full with fresh hot charcoal, and it got to ~ 200 and it stayed there for a ~4 hours hours until I pulled the meat. The meat on the chicken was still pink inside so I nuked it all finishing the job then refrigerated it.
I hope to get in the sealing gasket for the doors soon but I'm wondering about temperature control after I get the box sealed and charcoal brand going forward.
I figure there's some tweaking to do while the smoker is going but I'm not sure if part of the problem was the Embers brand charcoal or is this just a case of sitting there monitoring the temperature until I get the nuances of the smoker down?