The charge should be at least 7 grains, according to Alliant web site. This concludes that you use cavities of .66 cc or above in the measure. These big cavities should not result in any bridging even on shotgun powders, like Unique.
Make sure that your hopper is at least half full. Check that cave in your measuring disk is coming over centers of the proper tubes in "charge" and "drop" positions. Slightly hit the hopper when disk is on each ends and drop position - if it helps, considering the big disks that you use, there should be some imperfection on a way that powder comes to the hopper. I used graphite lube to prevent static electricity to stick the powder. I did not measure how did it improve the consistency though.
If nothing helps, try to check consistency of the measure with something that has smaller grains and is heavier. Salt and sugar are heavier - i.e. not affected by static, and have small grains. Corncob usually has smaller grains, but is light, and is affected by static electricity. It may help to identify where the problem is.
My measure with .66 disks and above drops any type of powder (ball, flake and stick) within less than 1.5% of standard deviation at worst, and usually well within 1%. Max spread, according to statistic formulas, is about triple of the standard deviation when you measure over 300 samples.
