Ok, I'll give it a go. Others will probably have differing opinions so YMMV, but this is what I have found.
The restrictor plate on the fan acts as kind of a global restrictor to limit the total air introduced into the system. I run mine quite closed down to 1/4" to 3/8". Any more open than that and I struggle with tuning it properly as that fan just creates too much air flow and pressure to gasify consistently.
The top primary plate limits the air flow into the firebox which will determine your rate of primary burn. With dry small splits it's surprising how little air is needed to keep things burning and coaling properly. I am burning very dry wood that has been inside the shed as well as some structured lumber cutoffs, so I have mine closed nearly all the way. It's a round plate covering a larger rectangular hole so the airflow is still there. I found it's very easy to add too much air into the primary firebox and blow the coals out of the nozzle and create a big hole and bridge. With a gassifier you need to be running an oxygen starved environment in the firebox to create the flammable gasses for the secondary chamber. Of course that also requires dry wood or the moisture will create too much steam in the primary firebox and kill the temperatures required for the secondary burn. If you are just blasting the flames into the secondary chamber using the air force of the firebox airflow, you're just running a gasser like a conventional and it won't work correctly. You'll get good gasification for the first bit before the wood starts to burn fully, then it will stop gassifying as all the gasses are being burned in the primary chamber, it will burn a hole above the nozzle and start bridging. I think too much oxygen in the primary chamber burns all of the coals and wood around the nozzle fully as the air is going from the primary to secondary chambers.
The lower plate restricts the secondary airflow into the reaction chamber. I have this set to 5 turns out. This one doesn't seem quite as picky to maintain gasification, but 5 turns for me is plenty to supply enough oxygen for the secondary burn. Any more and it just increases the stack temperature and blows coals up the tubes.
Keep your nozzle clean. Maintain a small coal bed, but not so much coal and ashes that the wood sits too high and creates a potential channel to burn out over the nozzle. Anything more than 2-3" and you're creating the opportunity for the wood to tent over the nozzle and not fall down so it's touching the nozzle and refractory because it's being supported on the sides by the ashes and charcoal. Stir the coals up once a day to bring the charcoal chunks up so they can burn so it doesn't get too deep. I clean the ashes out every week.
Long burns are actually ideal with gassers if they are tuned correctly. If it's gassing well at the beginning of the burn but stops after 10-15 minutes, I suspect the problem is too much air as the wood starts to burn too much in the primary chamber and it wants to burn like a conventional. I have mine tuned well and turned down to a slow enough burn that I am seeing stack temps of only 250 degrees with a good secondary burn and the nozzle glowing cherry red. It can burn this way for hours at a time and maintain gasification. Our natural inclination is to think that more air = hotter, cleaner burn = good. The light came on for me when I realized and accepted the fact that it's really counter-intuitive that you want to starve the fire of air to limit the rate of burn and make it burn cleanly. The nozzle is too small and restrictive to support a full burn in the firebox like you want from a conventional.
What does a typical burn cycle look like?
When does it gasify and when does it not?
What is your differential set to?
How often does it cycle?
It still sounds like your wood might be too high in moisture content. Can you post or PM me some pics what your coal and ash bed looks like as well as the "tenting" to see if that can help with further troubleshooting? Can you send pics of your logs and splits?