Well, I finally got to test the system last night after getting the barrel sealed up and am getting the same results, more shavings in the bag than in the drum. Did some more testing and found that if I close the 6" blast gate at the jointer halfway, the problem goes away. I also found that I am not having the problem when using my planer which is connected to the system using a 6" to 4" reducer. This leads me to believe that I need to go back to the stock 4" port on the jointer to reduce the amount of air being pulled into the ducting. I'll have to do the same on my table saw since I noticed last night that when the 6" blast gate on my TS is wide open, shavings are getting sucked out of the drum and into the bag even if I'm not cutting anything.
Give Laguna a call for assistance as they may well have some suggestions for you, including items like checking the impeller gap in the fan housing and may be able to help you with your problem. Part of the benefit of going with a reputable brand over a no-name brand is that there does exist a company to stand behind the product after the sale. Hopefully a call to them will help you get things straightened out without too many dead ends.
Best of luck!