I raised the gear off the ground so I could check everything. The right gear was sticking for two reasons:
1. The backing plate was fully inside the brake drum. I'm sure that was the way it was detailed when I installed the brakes, but a look at the most recent Sonex instructions for the brakes says specifically that the plate is NOT to be fully inside the drum.
So I added large washers to space the backing plate out.
2. Second, the arm (photo 2) that activates the brakes, was binding. I took it apart, re-set the angle so the brake cable come straighter to the arm, lubricated, and put it back together.
I took the opportunity to re-check the security of the wheels. The left gear had a little movement on the axle, which Sonex said to avoid. I used various thickness washers to get it right.