Start checking variables one at a time. With all the ash removed from the wsm and with all new briquettes, try your next cook with the stacker removed. If that solves the problem you can isolate why that is.
You can check the briquettes you are using. Maybe they are producing more ash than another brand and blocking and restricting air flow to your fire. I use mostly Kingsford competition and other than the occassional shake of the coals, I typically don't have an excessive amount of ash build up. You could also look at the source of your briquettes and see if changing helps.
You could also look into the guru adapter position and see if that makes a difference as well as to make sure the other vents are closed during your cook with the guru and that the adapter is sealed up well. With the curve of the bowl it can be difficult to get the adapter set right.
On my 10cfm fan I actually have the damper set to 50%, not sure if that makes a difference.
You could also check to see if your probes are properly calibrated - I'm sure they are but try the boiling water test and freezing water test as well to see. Additionally if you have a maverick or other remote thermometer, set probes for the maverick and the guru near eachother with the guru on but blower not installed and not controlling the pit and with your smoker manually going and check to see what readings you get and compare them to the guru's pit reading as well. If you are using the dome thermometer as a guide or reference, I have found it to be wildly inaccurate.
You could also try to manually take the pit to temp without the guru blower installed and see if you are able to get the temp up - if you can't you know that the problem is not the guru.
In the end we are trying to see the effects of fuel and air to control the temp and I'm sure a few tests can help you dial in. With the guru you should be able to set it and forget it, I know I do, but there have been occassions where I have made mistakes and it showed up with not achieving the temperatures I wanted.