blinkman wrote: Thanks very much for the suggestions!
To my astonishment (and no small amount of embarrassment) I discovered I had in fact installed RN2 the wrong way around. I am completely amazed to see that I had missed this again and again! (This does terrible things to one's confidence as a builder...)
But it's good news to have found the (likely) cause. Unfortunately I wasn't able to remove the thing without destroying it, so I've ordered a replacement and will follow up here after it gets here and I've installed it.