CrossRoads:
Okay, so given that things appear wired correctly:
What value is Rset?
Is Vcc at 5V?
Is the matrix installed upside down?
Rset is 18k. The device package says "1802", and on the two boards I assembled, I measure 18.1 and 18.8k. The 18.8 surprises me because they are supposed to be 1% precision resistors, but should still work fine.
I do have 5V on Vcc.
The first time I mounted the matrix to the wrong side of the board (actually the side I intended but I did get the header rows backwards on the PCB), the second time I installed on the opposite (electrically correct) side and I just double checked it's in the right way around.