I have not seen a schematic of that board.The GNDs on both sides of the board, would be convenient soldering pads to connect the commons between circuits.
Are you using TO92s?Maybe recheck pin out.
Let us check you wiring image.
I tried replacing with a couple of others. I could, of course, have 20 bad ones! Purchased x20 from eBay...I will re-build and photograph soon, maybe tomorrow evening.Here is the circuit with the 4-channel board.
So, can anyone suggest what I'm doing wrong?
Your average voltage measurements are pretty meaningless.
Get a scope.
Or write a solid '0' and '1' to D5 and see what you get.
I do not see a wire connecting the two black GND busbars.
That circuit is based on level shifters for I2C buses.See the attached application note.
...the 300R resistor... try removing that
I don't think that the FET you used is up to the job