One approach to verifying your build: ignore the wires, and power off, and use a continuity tester to verify firstly that every pin is connected to all the pins it should be connected to, and secondly that no two adjacent pins are shorted.

(A good choice of multimeter is one which has an audible beep for continuity testing, and ideally also a frequency mode.)

I agree with BigEd. Especially check power and ground connectivity.

_________________
Bill

This. I think the observed behaviour would be explained by the ground pin on U8 being unconnected.

...and I think you are absolutely correct! Much to my embarrassment - especially after my going on about having checked every connection religiously. I managed to miss this one.

Thank you everyone for all your thoughts and suggestions. And thank you kernelthread especially.