Inky phat not updating display

Adding to the pile of others who can’t get their Inky phats to update the display. I seem to have the same setup and symptoms as the others.

Installed fresh Raspbian distro (the full distro, not lite).
Tried several pi zeros and a pi 4b.
Very firmly pressed down onto the GPIO pins (as much as I am comfortable pressing).
Have re-seated the ribbon cable several times.
The GPIO pins are all soldered well on the pi (the pi4 came pre-soldered).
SPI and I2C interfaces are enabled.
Installation from the tutorial works.
Python examples all run without exception but no update is made to the display.

Any help is appreciated, of course. Hopefully this post at least increases visibility of the problem.