What sort of stems/bowls have been working for you? About how far from the tip of the heater are your herbs sitting? I'm still trying to figure out the best distance, and bowl/stem for it.
Mate...
sounds like you're running your GTR much too hot and with a load, that's probably too far away from the heater (so thereby creating a central hot spot, while the rest of the load keeps untouched). Additional screens in the heater shouldn't be necessary at all.
For the question re best bowls and distance of herb from the heater see below
Where's the screen positioned in that injector bowl!?
GTR-M (19mm version) is designed to utilize the tapered female joint itself for a bowl by placing a 17mm flat screen near the bottom of the taper, where it blocks securely. (Same with the 14mm version and a 1/2" screen)
The GTR injector port is shortened, so that it will only reach about 1/2 way into the female taper, leaving enough space between the screen (red dotted line in the pic) and the injector port (blue striped area in the pic) to place your herb.
If the screen is set much deeper (i.e. somewhere below the taper and within the body of the used adapter), the injector port might be too far away from the load and so needs higher voltage/wattage output for efficient roasting and might also take longer for reheating between hits.
Means, if you constructed your female 19mm bowl right, with that 17mm screen in the joint, your herb is usually only a couple of millimeters away from the male injector joint, when inserted.
I'd start with 9.5v from there and slowly turn up the voltage in 0.5v steps, till you find your temp, where it all starts vaping evenly.
Also, turn your GTR on at least 10-15 min, before you wanne use it, or best wait, till the bamboo shell is warm to the touch. It needs to reach a kind of thermal equilibrium, to work best.
Once this thermal equilibrium is reached (i.e. no more additional heat is sucked up by the log's body), you might even want turn down the voltage a bit, lest it gets too hot.
And then just leave it running. That's what's it designed for.
When I work from home, I switch it on in the morning and then leave it running and use it. Turning it off only, when I go to sleep or leave the house.
ETA: Can't say though, if it'll all work out now for you, as with the additional screens you added to the heater port there, we're now in uncharted territory. That was maybe a bit rash
ETA2: Did you get a VVPS (variable voltage power supply) from Alan with the GTR? Or are you running that thing on a continuous 12v straight!?