My smoker kept dying on me during long cooks, and I was ready to toss it.
I was so fed up when it would cut out halfway through a brisket, wasting hours of work. I mean, I checked the fuel and the thermometer, but it still happened. Then I noticed the air intake was clogged with old ash and grease. After cleaning it out real good, the heat stayed steady for the whole cook. Now I make sure to clean the intake before every long smoke. It fixed the problem and saved me from buying a new rig. Maybe it's just me, but that simple check made all the difference.
My old Weber bullet smoker did the same thing until I started taking the whole lower bowl apart. A thick layer of compacted ash builds up under the charcoal grate and blocks those bottom vents completely. Now I use a cheap putty knife to scrape it clean after every two or three cooks.
Used to just brush off the top ash and call it good. Then my temps kept dropping halfway through a pork shoulder. Took the whole thing apart like you said and found a solid gray brick under the grate. No wonder my fire kept choking out. That putty knife trick is a game changer for airflow.