Diagnosing Leburg ignition problem

[TaylorMono12]

I have installed Leburg dual ignition on my VW rebuild and all is wired up correctly - using the internal sensor version.

I have a healthy voltage, fully-charged batteries and can get nice big fat sparks on the A ignition. However, I'm getting nothing from the B ignition.

With the help of other Leburg owners, I've tried a number of things:

Ruled out the coil packs as the issue - I can get the B pack firing nicely by using the A coil loom
Ruled out the coil looms as the issue - I can get the A pack firing by swapping the coil looms entirely

Someone suggested that there might be issues with the terminals of the coil pack whereby one of the pins doesn't seat correctly, but I've looked under the magnifying lens and all looks fine.

My suspicion is the B controller itself BUT I'm getting the correct response from the LED / warning buzzer to show it's live and has correct voltage. The controller has been stored and handled correctly throughout.

My next step is to pull the controller off the back and try a different controller in its place (borrowed from another a/c) to see if that perfoms.

Any other ideas?