Can you post a pic of where you have the stove meter?
I'm trying to imagine why you would be able to get a robust fire with good secondaries at some times, then not be able to duplicate it later, if you are using the same startup procedure. Is your house super-tight, where you might on occasion have a dryer or kitchen vent competing with the stove for air? I know you tested your wood and got good readings, but could it be that some of your splits are drier than others? Do any of the splits hiss when they have heated up on the coals for several minutes?
If you have the air wide open, you may get a lot of flame but the heat will be going up the flue (which might overheat.)
Maybe you can describe your startup procedure..air settings, amount of time, and how much, when you cut the air back in stages, stove temp readings, etc.
As suggested in your other threads, you really need to get the flue probe in that connector pipe. It will give you much more immediate feedback regarding temp conditions in your stove, than a stove meter will. This will tell you when to cut the air, what temp you need to get to and hold when you are starting a new load in order to get good secondaries, and so on. It will also keep you from overfiring your pipe.
I run my flue meter up to X temp, cut air and hold it there for 10-15 minutes, then I'm good to close the bypass and cut the air to my final setting in a couple of steps.
I would also get an IR thermometer so that you can shoot through the top of the convection chamber to your stove top. I got a re-badged Fluke for not too much, but others may know of a good cheapie you can pick up.
Is your Manchester an 8630 or 8631? I remember seeing in the 8630 materials a couple years ago that they listed only 10' of chimney needed for the stove, so you should be OK there. Or maybe
@webby3650 knows if any changes were made from the 8630 to the 8631?
Disclaimer: I've got a cat stove, maybe some of the tube burners here can offer operational tips that will better apply to your stove. Or you can just start searching and reading topics if you have a few months to spare.
It takes a little time to figure out what works in your particular situation, but you
will get there.