When your secondaries kick in, and you start to close the air down , does the glass clear, even somewhat?
It ties into what Woodsplitter said, above.
Normally the bottom sides darken some. But the middle stays clear.
I’ve been trying to extend burn times and have been more aggressive in turning down the air. Maybe too much so? It also possible I’m not reloading very efficiently…putting in too much all at once with too little coals, or activity to catch it quickly?