Hmm........alot of very good analysis posted above.
I would have to ask this: was the fat cap trimmed off pretty thin? Looking at the pic, it looks like you started out pretty lean...that should be a benefit when trying to achieve better smoke penetration.
I'm not sure about the ambient humidity being an issue, as I live in a fairly arid climate, but always begin my smokes with a humid smoke chamber...and maintain water until all my meats are smoked and foiled.
I have noticed with my briskets that the smoke ring does not form very deeply, but it has a very prominent color for about 1/8" to 3/16". I get very deep smoke rings with pork, but not beef.
I do agree that a high chamber temp could very well be the culprit. If I have chamber temps of 250*+, instead of ~225* for the first couple of hours, my smoke penetration is not as deep and there is a noticable difference in flavor.
With that, I would go with the advice above, and put a probe on your rack and double check the door gauge with that thermo, and possibly have a couple of oven rack therms in the chamber on a 2 other rack positions for a good reference.
If your GOSM door therm is like mine, it has a a hex "nut" on the therm stem and can be calibrated...keep in mind that mechanical thermometers generally have their best accuracy in mid-range, so that's where you want to do the calibration checks/adjustments.
Keep trying Ron, brisket makes a mighty fine meal when you can get it the way you like it...I know, 'cause I hooked on 'em now.
Good luck & good smokes!