With 800* + insulation the ambient temp shouldn't affect the cooking temp too much,unless the door is opened a lot.
I assume you have checked the actual temp at the cooking spot in the cooker with correct remote therms and your instaread therm was also calibrated?
If so,you might give CS service a call.
Packers can hit a plateau around 185*,but as it closes in on 190*,it will increase in internal temp pretty quickly.
If you thought the briskets were good at 185*,another 10* might make them even better.
If all the above is correct and time and your max temp won't let you get to a higher internal temp,you can always try the dreaded foil.
I'm not advocating it as a standard practice,but it can lessen cook time and increase internal temps.