Two things to deal with, the electrical part and the mechanical.
If the wires were original to the brake locations, are they wired in a star configuration or are the primary brake wires daisy chained from one to another. The star type is superior, IMO. 12 on the Prodigy should be enough to move all the magnets to the drums. IIRC, my Prodigy II shows 4 or so during regular braking. I don't use the boost function. I don't know if measuring current when the wheels are up will turn up a malfunctioning brake but it may turn up a difference in that one bad location. If it was the same as others, that might eliminate the electrical side as a potential issue.
If the drums aren't worn to a large degree and the shoes have adequate pad that isn't glazed, maybe adjusting them tighter may improve the braking. FWIW, I adjust mine until they are lightly scuffing and even a little resistance, at times. It depends on the age of it all and what the condition is of the contact area. Another alternative is to replace the shoes, they are still rather cheap.
AFAIK, the lever with the magnet should have very little resistance. Maybe you have one that is caught up?